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(1) Introduction

• What is brackish water?
• Estuarine and low salinity environments and habitats
• The basic equipment for a brackish aquarium
• Common and not so common brackish water fishes
• What fish can’t be kept in a brackish water aquarium
• Are there any books on the subject?

(2) Setting up the brackish water aquarium

• Artificial sea water and measuring salinity
• Water chemistry and temperature
• Substrates and tank decor
• Planting the brackish water aquarium
• Healthcare and medication
• Breeding brackish water fishes

(3) Commonly available fishes

• Monos (Monodactylus spp.)
• Scats and Argusfishes (Scatophagus and Selenotoca spp.)
• Archerfishes (Toxotes spp.)
• Targetfish (Terapon jarbua)
• Brackish Water Pufferfish (family Tetraodontidae)
• Cichlids (family Cichlidae)
• Mollies (Mollienesia or Poecilia)
• Rainbowfishes (family Melanotaeniidae)
• Silversides (family Atherinidae)

(4) Predatory fish

• Gar (Lepisosteus and Atractosteus spp.)
• Colombian shark catfishes (family Ariidae)
• Sleeper gobies (family Eleotridae)
• Siamese tiger fish (Coius or Datnioides spp.)
• Ropefishes (Erpetoichthys calabaricus)
• Freshwater moray eel (family Muraenidae)
• Bullrouts (Notesthes robusta)
• Flatfishes and soles (family Soleidae and others)
• Spiny eels (family Mastacembelidae)
• Needlefish (family Belonidae)
• Butterfly-goby waspfish (Neovespicular depressifrons)

(5) Oddball fish

• Four-eyed fish (Anableps spp.)
• False four-eyed fish (Rhinomugil corsula)
• Mudskippers (Periophthalmus and Boleophthalmus)

(6) Small fish

• Killifish (family Cyprinodontidae)
• Bumblebee and other gobies (family Gobiidae)
• Glassfish (family Ambassidae, formerly Chandidae)
• Pipefishes (family Syngnathidae)
• Livebearers (family Poecilidae)
• Halfbeaks (families Zenarchopteridae)
• Blennies (family Blennidae)
• Spaghetti Eels (Moringua spp)
• Glass Eels and Rice Paddy Eels (Pisodonophis spp)

(7) Marine fish

• Introducing marine fish to brackish water aquaria
• Marine puffers (Arothron hispidus and others)
• Batfishes (Platax spp.)
• Snappers and Sea Breams (Lutjanidae/Sparidae)
• Damselfishes (family Pomacentridae)
• Eel-catfishes (Plotosus spp.)
• Seahorses (Hippocampus spp.)
• Pike-conger eels (Congresox talabonoides)
• Bar-tailed flathead (Platycephalus indicus)

(8) Invertebrates and native brackish water fish

• Tropical brackish water invertebrates
• Tropical brackish water snails
• Native brackish water fishes and invertebrates

Native Fish & Invertebrates

Native species of fishes obviously depends on where you live, but among fishkeepers the term is generally used to indicate temperate water species as opposed to the tropical species that make up the bulk of the ornamental fish trade. Brackish water species of goby, blenny, stickleback and flatfish can all make good aquarium inmates provided their basic requirements are met. The main issue is temperature. Temperate water fishes are used to low and moderate temperatures, and prefer a temperature around 15 to 18° C. The following are European species that should do well.

Gobies

The sand goby Pomatoschistus minutus and the estuarine gobyPomatoschistus microps are two hardy European gobies that will do well in aquaria. They can be found in great numbers in estuaries and rock pools from Britain and Scandinavia down to the Mediterranean and Black Sea. Both grow to about 10 cm (4 inches) and are attractively patterned and will readily feed on small frozen foods as well as insect larvae, brine shrimp and similar live foods. They appreciate a soft sandy substrate for burrowing into and being somewhat territorial it is a good idea to provide them with plenty of sea shells or artifical caves like flower pots. A specific gravity of 1.010 and upwards is ideal. Aeration of the water is essential particularly in summer when the temperature may become somewhat higher in an indoor tank than in their natural habitat, but otherwise these are robust and hardy fishes. I have come across these fishes quite commonly mixed with live river shrimps and sticklebacks being sold as fish food. Gobies are often suprisingly good at jumping out of uncovered aquaria.

Blennies

The peacock blenny Salaria pavo is a brackish-water species found along the Atlantic coastline of North Africa through to the Black Sea. It will tolerate freshwater and marine conditions but does best in brackish water. It is a hardy, active predatory fish that will eat most foods and has proven to be easily maintained in aquaria, but it is apt to be aggressive and territorial towards other fish. Like gobies, this fish needs lots of hiding places and a securely covered aquarium.

Two temperate zone blenny species should be mentioned with regard to fishkeeping. One is the shanny, Lipophrys pholis. This rockpool dweller is an excellent aquarium fish, notably for its adaptability and hardiness. It does very well at room temperature, though in summer it will crawl out of the water to cool down on top of any conveniently placed rocks or filters. Although the shanny is undoubtedly able to tolerate short-term changes in salinity given its habitat, it isn’t a brackish water fish and shouldn’t be kept in a brackish water aquarium.

Very occasionally, the freshwater blenny Salaria fluvialitis is seen in tropical fish shops. It is a mountain stream fish that needs cool, well oxygenated water similar to the conditions enjoyed by subtropical danios, loaches and barbs, with which it would mix very well. Although territorial, it is otherwise a good community aquarium fish. That said, it has no place in a brackish water aquarium.

Sticklebacks

Sticklebacks are widely distributed throughout the northern hemisphere. The three-spined stickleback Gasterosteus aculeatus is found in North America, Asia, North Africa and Europe in freshwater, brackish and marine conditions. They have been much studied by biologists in particular because of the ease with which their breeding behaviour can be observed in captivity. Sexually acitve males are very pretty, bright blue or green with shiny eyes and a deep orange or red belly. Females and males outside of breeding condition are a pale yellowy-silver.

Although widely distributed, individual populations appear to be suited to fresh, brackish or marine conditions, and the collector should set up an aquarium for these fish according to where they are collected. Essentially very hardy, though good filtration and well oxygenated water are important. They are distinctly predatory and should be offered frozen or live foods such as bloodworms, brine shrimps and water fleas.

Outside of the breeding season these sticklebacks are gregarious and essentially peaceful towards one another; during the breeding season males are highly territorial. Sticklebacks are best kept alone and tend to mix poorly with other fish species; they will often bully small fish, while larger fish may view them as food.

The nine-spined stickleback Pungitius pungitius is a more cold water species found in Europe, Asia and North America and needs a cool aquarium with excellent filtration and oxygenation. It can be found in fresh waters but is primarily a brackish water and coastal marine fish. The fifteen-spined stickleback Spinachia spinachia is a strictly brackish and marine fish that can be found in rock pools, sea grass meadows and similar habitats where it looks and behaves rather like a pipefish, sneaking up on the small animals it eats. It requires a similar cool, well filtered tank to the nine-spine stickleback.

Flatfish

The common European plaice, Pleuronectes platessus, when small makes a good inmate for a cool-water strongly brackish or marine aquarium. It is hardy, tolerant of room-temperature water, and not at all shy. It will readily accept non-living foods once settled in, even catfish pellets. The European plaice grows quite large though, up to 100 cm (36 inches) in length, and can live for fifty years.

The flounder Platichthys flesus is similar to the plaice but smaller and tolerant of a wide salinity range, wild fish being commonly found in freshwater rivers miles from the sea. Maintenance is similar to the plaice, though this species tends to be a bit shyer and a little more delicate under aquarium conditions.

Invertebrates

Numerous small invertebrates can be added to a native brackish water aquarium, but needless to say the exact species will depend on where you are collecting them. Shore crabs, Carcinus maenas, are extremely hardy even of tropical temperatures and will adapt to any marine or brackish aquarium quickly. Adults will tolerate a range of conditions from normal marine to only slightly brackish (a specific gravity of about 1.004), and will thrive in anything from half to full strength seawater. These crabs do get big, aggressive and predatory though and so usually only small specimens are suitable for aquaria. They are also apt to come out of the water when warm, and will escape from an aquarium if they can.

Much more suitable is the small estuarine mussel crabs of the genus Pinnotheres. These never get bigger than one centimetre (0.4 inches), and only the males are likely to be seen moving about. The females are larger, red in colour, and live inside mussel shells. Small hermit crabs may also be found, and if from brackish water environments, kept in an aquarium easily. As is well known they use old snail shells for protection, and do need a range of successively larger shells to choose from as they grow.

Small shrimps such as the brown shrimp Crangon crangon and the common prawns Palaeomon elegans and Palaemon serratus are also very hardy but likely to be eaten by substantially bigger fishes. They will do fine with the smaller gobies though.

Tropical Brackish Water Snails

Snails are often added to the aquarium for algae control and as scavengers. In the case of pufferfish, they might also be used as live food. Some snails are highly beneficial, others less so; it is important to choose and use snails carefully.

Snails (non-nerites)

Snails are popular with aquarists because they tend to be hardy, easy to keep and generally harmless towards other livestock (except soft plants and fish eggs). Most freshwater snails, such as apple snails, are not salt-tolerant and have no place in the brackish water aquarium.

Melanoides tuberculata is known among biologists as the red-rimmed melania, but aquarists usually refer to this small freshwater snail as the Malayan livebearing snail or Malaysian trumpet snail (often abbreviated to MTS). Though not large (typically around 3 cm/1.2") these snails are certainly attractive in their way, with a sturdy, spire-shaped shell bearing a delicate pattern of spiral grooves. Adult snails are usually marked with reddish-purple bands and spots, particularly on the final whorl of the shell. The closely related quilted melania Tarebia granifera is sometimes mistaken for the Malayan livebearing snail. Its much more globular rather than spire-shaped shell and the absence of colour banding easily distinguishes it from the Malayan livebearing snail. It does not naturally occur in brackish water and has minimal tolerance for brackish water in the aquarium. Another similar species is the fawn melania Melanoides turricula that has the same tall shell as the Malayan livebearing snails but lacks the red markings. Again, this species isn’t a resident of brackish water and is more normally found in clean, fast-flowing streams.

Malayan livebearing snails are essentially hardy animals that pose no problems to the aquarist in terms of maintenance. They will accept a very wide range of water conditions, from slightly soft and acidic through to fairly strong brackish water (at least SG 1.012). Water temperature does not matter greatly, and they will accept anything from 18˚C to over 30˚C. One of the more remarkable aspects of the biology of these snails is their ability to enter a period of dormancy is conditions become hostile. In the wild they will hibernate through cold periods and aestivate through droughts, sealing the aperture off to prevent moisture loss. There are many reports of Malayan livebearing snails surviving in buckets of gravel or sand for months at a time, only to spring back into life as soon as that gravel is put into a new aquarium.

Curiously for such attractive and easily maintained snails, opinion is sharply divided as to whether these snails are a blessing or a curse. To their credit, these snails never touch live plants. They burrow through the substrate keeping it oxygenated and clean, eating any organic matter they can find. They are especially good at preventing organic matter from decaying anaerobically in sandy substrates. On the debit side of the equation is the fact that these snails can breed very quickly if there is plenty for them to eat. So while plagues of these snails really say more about poor aquarium maintenance than anything else, they do have a reputation for being a pest.

Pachymelania byronensis is a West Africa snail that is periodically sold for use in freshwater aquaria. These are impressive snails, having shells that get to about 6 cm/2.5" in length and are covered in short but sturdy spines. In the wild this species inhabits the upper part of estuaries where it can be found in either freshwater or brackish water to a salinity below about one-quarter that of seawater (in aquarium terms, 1.000 to 1.005). At least two other species of Pachymelania are known, and these are found further down the estuary in saltier water. All Pachymelania are believed to have planktonic marine larvae, so breeding these snails in aquaria is likely to be difficult, perhaps impossible. These snails appear to be more omnivorous than herbivorous, and they ignore plants and eat little algae. On the other hand, fish foods of all types are readily accepted.

One final thing should be mentioned about the Pachymelania snails: they are known to carry parasites transmissible to humans. As such, they should be quarantined and treated before being introduced to your aquarium. Check with your retailer whether this has been done before buying them, and if in doubt, consult with your medical practitioner. Nerites and ampullariids are not known to carry dangerous parasites, and in fact ampullariids are used to exterminate such species of snails by eating their eggs and out-competing them for food, so these can be used safely in any aquarium. Wild melanias, including the Malayan livebearing snail, have been reported to carry parasites, but because these parasites need to pass through a variety of hosts such as fish and birds that eat the snails, to complete their life cycle, aquarium populations pose no risk at all.

A more problematic addition to the brackish water aquarium is the Colombian ramshorn snail, Marisa cornuarietis. Despite its common name, this snail is in fact a member of the apple snail family Ampullariidae. Most ampullariids are rather intolerant of salt, but Marisa cornuarietis is an exception and naturally occurs in both fresh and brackish water environments. In an aquarium it can be expected to do well at a salinity of up to about one-quarter strength seawater (SG 1.005) though rather less would be preferable. As such, it could make a useful and geographically authentic addition to a low-end brackish water aquarium containing Central American livebearers or killifish such as mollies and Florida flagfish.

Snails (nerites)

Nerite snails feed almost exclusively on algae, predominantly diatoms and green algae, but in the case of the American zebra nerite at least, blue-green algae as well. Most species are found in saltwater habitats, but many live in tidal areas including brackish water habitats, and a few are exclusively freshwater residents. Nerite snails are perhaps the ideal algae-eating animals for the brackish water aquarium, with the only caution being that some fish, notably puffers, will view them as food. Nerites tend to be quite small, typically no more than 15 mm in shell width, so you should allow at least 2-3 per 45 litres/10 gallons if you want them to use them for algae control. Nerites do not breed in freshwater tanks since most species have a saltwater larval stage, but even in brackish water tanks it is very unlikely nerite snails will succesfully breed, though many species quite happily lay their eggs on solid surfaces.

Various freshwater nerites have become commonly traded in recent years, and most should do well at low salinities (SG 1.005 or less). These include the polka-dot or orange-track nerite Vittina semiconica; the African zebra nerite Neritina natalensis; and the Indo-Pacific zebra nerite Vittina coromandeliana. The freshwater limpet Septaria porcellana is another nerite snail, and can be expected to adapt to low salinity systems below SG 1.005.

Some nominally freshwater nerites are known to inhabit areas with tidal or brackish water influence, and should adapt to a broad range of brackish water salinities up to around half-strength seawater (around SG 1.010). These include the spiny nerites such as Clithon corona and Clithon sowerbyana, and Batman snail Neripteron auriculata.

Other nerites are primarily marine species that may occur in brackish or freshwater habitats periodically. These will do best at middling to high salinities, SG 1.010-1.025, even if sold as freshwater nerites, as sometimes happens. Examples include the Virginia nerite Neritina virginea, the American zebra nerite Puperita pupa, and the Indo-Pacific chameleon nerite Nerita chamaeleon.

Tropical Brackish Water Invertebrates (other than snails)

Although there are many invertebrates to be found in brackish waters around the world, only a very few of these are traded. The following are the ones most frequently encountered in tropical fish stores. Some, such as the blue-legged hermit crab and Asian horseshoe crab are more likely to be sold as marines, while fiddler crabs and nerites commonly get sold as freshwater animals. When adapting these organisms to brackish water, care should be taken to acclimate them to the new salinity careful. In addition, most invertebrates are intolerant of pollutants, including nitrate and copper.

Clams

Bivalve molluscs do not usually do well in aquaria. They are filter-feeders and cannot get enough to eat without regular feeding of a suitable invertebrate diet (sold to marine aquarists for feeding corals, tubeworms, etc.). The freshwater Asian clam Corbicula fluminea is one of the most commonly seen. They can reproduce in captivity, releasing larvae that are able to crawl about at once. This species will tolerate very slightly brackish water, to around SG 1.003-1.005.

Fiddler and red-claw crabs

Fiddler crabs, Uca spp are small crabs in various shades of yellow and brown, but the males have one enormously developed claw with which they display. These are semi-terrestrial animals and will not adapt to permanently being kept underwater. They need rocks, a sandbank, or some other sort of structure above the water level onto which they can climb and feed. Ideally, keep in a mudskipper style aquarium. Fiddler crabs feed on organic detritus, primarily algae and decaying plant material.

Another semi-terrestrial freshwater crab is the Asian red clawed crab Perisesarma bidens. It is found in mangrove forests and will dip into fresh, brackish and marine water. It requires similar conditions to fiddler crabs and cannot be kept permanently submerged. Maintenance of this species is not difficult, but it is an opportunistic predator and will eat small fish.

Crayfish

None of the commonly traded species of crayfish are brackish water animals, and so do not have a place in the brackish water aquarium.

Hermit crabs

Various Clibanarius species are found in tidal and mangrove habitats, and these can do well in mid to high salinity brackish water aquaria, corresponding to specific gravity levels of 1.010 upwards at 25 degrees C. Two species from North American are quite widely sold, Clibanarius vittatus (the common or green hermit crab) and Clibanarius tricolor (the Caribbean or blue-legged hermit crab). Both are hardy, easily maintained, and safe with tankmates of similar size. They will fight over shells, so it is important to provide lots of suitable snail shells that they can use as they grow.

Clibanarius longitarsus is A brackish water hermit crab from Southeast Asia that requires similar conditions to the North American species mentioned above. The so-called freshwater hermit Clibanarius africanus is an African species does best in low to middling salinity brackish water, SG 1.005-1.010 being ideal.

Terrestrial Hermit crabs

At least two terrestrial hermit crabs are sold as pets. These are the Purple Pincher hermit crab (Coenobita clypeatus) from the Caribbean and the Ecuadorian hermit crab (Coenobita compressus) from the Pacific coastline ofSouth America. Neither can be kept in an aquarium setting, since they both spend almost all their time on land. However, they might be kept alongside mudskippers and fiddler crabs assuming that these were all of similar size. Note that while Coenobita clypeatus will happily bathe in brackish water, it needs access to freshwater for drinking. Both species will drown in deep water, so it is important that they can climb out of the watery side of the tank easily, perhaps via a sand bank or sloping pieces of bogwood.

Horseshoe crabs

Horseshoe crabs (really an animal more closely related to the spiders and scorpions) are large, burrowing, animals occasionally sold as oddballs for tropical marine aquaria. The American species, Limulus polyphemus is the best known and most widely traded, and though tolerant of brackish water for extended periods, probably needs fully marine conditions over the long term. The same is probably true for its close relatives, the East Asian horseshoe crabs of the genus Tachypleus.

On the other hand, the more distantly related South Asian horseshoe crab Carcinoscorpius rotundicauda is very much an estuarine animal and has even been found in freshwater environments, such as 90km up the River Hooghly (Chatterji, 1994 in Xuhua Xia, Systematic Biology, Vol. 49, 2000). Because of this, the South Asian horseshoe crab likely makes a much better choice for the mid- to high-salinity brackish water aquarium. It is known to be very hardy, providing it is given plenty of food, and does need a sandy substrate so that it can burrow freely.

Jellyfish

The mangrove jellyfish Cassiopeia andromeda is tolerant of a range of salinities from about half-strength sea water upwards, but is very particular about substrate and lighting. A soft sand substrate is essential as these animals spend most of their time upside down settled on the sand with the tentacles upwards to catch food. They also need intense light because like corals they possess symbiotic algae. Also like corals, absolutely perfect water conditions are required, so that generous filtration and regular water changes will likely need to be supplemented with protein skimming.

Sea anemones

While there are brackish water anemones, only one is currently traded as such, the beadlet, Actinia equina. This is a hardy, coldwater species that does well even at tropical temperatures provided water quality is good. It is not photosynthetic. In the wild these anemones are not normally exposed to very low salinities for long, and in the aquarium should probably be kept at SG 1.015 upwards, and it is questionably whether even that level of salinity will do them much good over the long term.

Shrimps

The Amano shrimps have become popular as algae eaters in freshwater aquaria, although they will do well in slightly brackish water as well (and in fact only breed in brackish water). They are small and can only be kept with very small fishes of similar size, such as bumblebee gobies, that aren’t going to bother them.

A true brackish water denizen is the red-nose shrimp Caridina gracilirostris. It is similar to the Amano shrimp in 

shape and colour except that it is more slender and has a long, red rostrum (“nose”). Specimens kept in freshwater aquaria tend to be a little delicate, but in tanks with a specific gravity around 1.003-1.005, these shrimps are much more robust. They are a bit more omnivorous than Amano shrimps, and though they eat algae, meaty foods are appreciated as well.

Various species of Palaeomonetes are inexpensively sold as "ghost shrimp" or "river shrimps" and these will do well in fresh or slightly brackish water (specific gravity below 1.005) making them ideal companions for very small fishes such as bumblebee gobies, small livebearers, and other inoffensive animals. They are all scavengers to some degree, although algae in an important part of their diet and should be made available, perhaps by not scraping the walls of the aquarium.

Though not often sold as an aquarium animal per se, mention has to be made of the Hawaiian volcano shrimps sold in “self-contained ecospheres” as novelty gifts. Contrary to the advertising blurb put out with the ecospheres, these shrimps cannot survive simply by eating algae and consuming the oxygen the algae produce, but merely die very, very slowly. In fact these shrimps, known to science as Halocaridina rubra, are adapted to living in harsh environments such as coastal pools and streams where the salinity and temperature can change dramatically and quickly. It is their hardiness, rather than any clever design behind the ecospheres, that allows these shrimps to last for as long as they do. In aquaria they do very well in moderately brackish water (SG 1.010) under which conditions they will breed quite readily. At barely 1.2 cm/0.5" in size, these shrimps cannot be combined with most fish, which will simply view them as food, though possibly very small gobies such as Brachygobius aggregatus are an option.

Marine Fish in the Brackish Water Aquarium

A number of commonly traded marine species are actually found in brackish water in the wild. As such, they can make interesting temporary or even permanent additions to the brackish water aquarium depending on the species.

Before adding marine species to your brackish water aquarium, make sure that the water is well oxygenated and properly filtered, with no ammonia or nitrite and only low levels of nitrates (20 mg/l or less). The biological filter must be fully matured. Use a protein skimmer if possible (these work increasingly well in brackish water once the specific gravity reaches 1.010 or more) to minimise the amount of organic matter in the water. Water chemistry is important too, so ensure that the water is hard, alkaline, and effectively buffered against pH drops.

The best way to introduce brackish-tolerant marine fish to the brackish water aquarium is by using some sort of ‘drip method’ to gentle acclimate the fish to the ambient water conditions. Put the marine fish in a bucket along with the water it shipped in. Over the next 30-60 minutes add small quantities of water from the brackish water tank. Once the specific gravity of the water in the bucket approaches that of the aquarium, carfully remove the fish using anet and gently place it in its new home.

It is important to ensure the fish is feeding and well settled. In some cases, these fish are brackish water denizens only when young, and once they mature will need to be transferred to a normal marine aquarium.

Marine Pufferfish

Many of the marine puffers range into brackish waters, especially when young.

Arothron spp.

Arothron hispidus is a large but generally well mannered pufferfish that is usually imported as marine fish but is sometimes offered as a freshwater fish. Compared with the standard brackish water puffers like Tetraodon fluviatilis, Arothron hispidus is larger but generally less aggressive and altogether easier to keep with other fish. Provided you have the space for a fish that can easily reach 30 cm in captivity (and up to 50 cm in the wild), this species makes an excellent aquarium subject.

Juveniles may be collected in estuaries or even in completely freshwater conditions, and this is why they are sometimes offered as freshwater puffers. Kept this way they never really do well and are prone to disease. Juveniles sold as freshwater fish should be immediately transferred to a brackish water aquarium at SG 1.005 or more. They can of course also be acclimated to fully marine conditions. In the wild these puffers migrate into the sea once they mature, but in an aquarium will do well at SG 1.010 or more.

Some of the other Arothron species also inhabit brackish water including Arothron immaculatus, Arothron reticulatus and Arothron stellatus.

Other Indo-Pacific species

Canthigaster compressa is one of the sharp-nose puffers and inhabits estuarine and marine habitats such as lagoons and harbours. It is fairly tolerant of low salinities but should not be exposed to water with a specific gravity of less than 1.010. It is a small, peaceful fish that exhibits outstanding colours: a reddish body with white spots below and blue stripes above, and a large eye-spot beneath the dorsal fin. Quite widely traded as a marine aquarium fish and an excellent companion for scats, monos, and other fish requiring strongly brackish to marine conditions.

Chelonodon patoca is a rarely seen Asian pufferfish that can adapt to freshwater for a while but really needs moderately brackish to fully marine conditions over the long term. It is a large species, growing to about 30 cm in length, but is usually peaceful towards its own kind though it should not be kept in overcrowded conditions. Initially this species was described as being peaceful (for example in Baesnch) but there are some reports that suggest this species can be a fin-biter. A second variety, known as Chelonodon ‘gold spot’ is in the trade as well; presumably it is similar to the milk-spotted puffer in terms of behaviour and requirements.

Takifugu species are increasingly widely offered as freshwater or brackish water fish, though their success in aquaria varies. The most commonly traded species is the peacock puffer Takifugu ocellatus, a metallic green species with bright orange markings. While offered as a tropical freshwater fish, kept in a tropical freshwater aquarium it is very short lived. This species comes from China where it is found in estuaries; consequently the best way to keep this fish is in a subtropical rather (15-18 degrees C) than tropical aquarium at a middling salinity (SG 1.010). It is generally peaceful, though some specimens have been reported to bite other fish.

Another Takifugus species offered is the Japanese pufferfish Takifugu rubripes, yet again a subtropical/coldwater species from the Far East. Lab work suggests this species cannot be kept in freshwater indefinitely, but in mid- to high-salinity brackish water systems it is reputed to be quite robust.

Atlantic species

Colomesus psittacus, known as the parrot pufferfish, is a very large (30 cm) South American species that is rarely imported as an aquarium fish. Physically it resembles the popular South American freshwater pufferfish, Colomesus asellus, but is bigger and lacks the black band encircling the base of the tail fin. It is known to be peaceful, though given its large size it must be assumed to be potentially predatory towards smaller tankmates. May be offered as a freshwater fish, over the long term this fish should be considered a high-end brackish to marine species and kept in the same way as Arothron hispidus.

Sphaeroides maculatus is a subtropical species found in the North Atlantic and sometimes known as the northern puffer. It is quite common in shallow estuaries, oyster beds, and other brackish water habitats and aquarists in places like Florida can easily find and catch these fish. They are rather nervous when first introduced into the aquarium and will not do well in small tanks or with aggressive fish. But they are quite hardy and adaptable, and will eventually settle down without problems.

Batfish

Note: There is an excellent FAQ on these fishes by Bob Fenner at Wet Web Media.

Although rarely thought of as brackish water fish, the natural habitat of juvenile batfish includes mangrove swamps. Their strange shape and mottled brown colours allow them to blend in with drifting vegetation in the rivers, camouflaging the little fish as dead leaves. When they grow up they move into more marine habitats, being common on reefs but also around harbours and other shallow water habitats. Kept in big aquaria with other large fish and they are completely peaceful and easy to tame, even friendly towards their owners.

Three species are commonly imported. All are rather similar as adults, basically round and silvery brown. They are most easily identified when the fish are young. All three batfishes are notable for getting big irrespective of the size of the aquarium, unlike many marine fish which tend to be smaller in captivity than in the wild. For this reason, do not buy a small fish hoping it will grow to the tank -- it won’t! Plan on a 350 litre (100 gallon) aquarium, at least, if you intend to keep these fish in a community setting with suitably robust species likes Colombian sharks, scats, and monos. Some specimens can be quite boisterous though, particularly at feeding time, so they shouldn’t be kept with timid fish. It is also important to remember that all are predatory, and will eat any small fishes or invertebrates that they can swallow, so they cannot be kept with small livebearers, gobies, etc. On the other hand, the long fins can be a terrible temptation for fish apt to nibble at such things, so it is not a good idea to keep them with pufferfish. Read the section on adapting marine fish to brackish water to find out how to adapt batfish to a brackish water aquarium. 

Platax teira

Platax teira is known as the long finned batfish. Juveniles look very much like freshwater angelfish, but are rather darker. The dorsal and anal fins are very long, and the pectoral fins are elongate as well. In colour they are a rather murky brown. Reaches around 70 cm (30 inches) in length when fully grown.

Platax orbicularis

Platax orbicularis is known as the round batfish batfish and although it resembles the long finned batfish when mature. The names comes from the juveniles, which are much more rounded and leaf-like, and often a lighter brown in colour. A bit smaller than Platax teira when mature, up to 50 cm (20 inches) long, but still a jumbo fish for correspondingly big aquaria.

Platax pinnatus

Perhaps the prettiest batfish, Platax pinnatus is known as the dusky, red-striped, or red rimmed batfish. The juveniles are similar in shape to the long finned batfish, but are much darker brown, almost black. The fins have a red or orange edges, and the face has some markings as well. Adults are not very different to the other species, and the red markings are lost. At 45 cm (18 inches) it is the smallest of the commonly traded batfish species, but still requires a very large aquarium. It is is said to be the most predatory of the batfishes, and even when small stealthily creeps up on small prey items, while the other species are less predatory and will adapt to dead foods more easily. Although an attractive fish, this species also has a reputation for being by far the most delicate of the commonly traded batfish species. It should not be kept except perhaps by experienced aquarists, and certainly not in the typical brackish water aquarium.

Snappers and Sea Breams

Both of these groups are predominantly marine group in distribution but species from both families are found in brackish (rarely fresh) waters from time to time.

Snappers (family Lutjanidae) are active and hardy fish, but their large size and predatory nature means that they are of dubious value as aquarium fish. Given sufficient space they are generally easy to keep alongside fish of comparable size, though some species are territorial and aggressive. Several species enter brackish water in parts of their range, including the following species periodically traded as aquarium fish:

• Lutjanus apodus, the schoolmaster snapper (to 60 cm in the wild)
• Lutjanus argentimaculatus, the mangrove jack (to 100 cm in the wild)
• Lutjanus sebae, the emperor snapper (to 110 cm in the wild)

Lutjanus argentimaculatus is a truly euryhaline species found regularly in fresh, brackish and salt water at all stages of its life. The other snappers listed here are usually only found in mangroves and estuaries when young, migrating into fully marine environments as they mature. Consequently those species will need to be maintained at a fairly high salinity to do well, certainly not less than SG 1.010.

Sea Breams (family Sparidae) are similar to snappers in some ways, but whereas most snappers feed on fish, sea breams specialise on hard-shelled prey such as crustaceans, though many species eat small fish as well. They are mostly marine in distribution, but some species also occur in freshwater or brackish water environments.

The fish illustrated above appears to be a sea bream, and has lived for several years in a freshwater aquarium. It has grown and is apparently healthy, and is currently on display at the Maidenhead Aquatics store in St Albans where it lives with an arowana and some stringrays. Its species is unknown, though it is likely a species of Acanthopagrus such as Acanthopagrus berda or Acanthopagrus latus. Sea breams are generally active but peaceful fish that work well in communities alongside similar sized fish.

Damselfish

There are many different damselfishes, but very few enter brackish or freshwater. They are primarily fishes of tropical marine waters around bays and reefs. Damselfish are very closely related to cichlids, and share many of their vices and virtues. They are hardy, active fish which can be easily tamed and adapt well to captive life taking most types of food readily. They are also excellent dither fish, helping nervous fish like monos to settle in. On the other hand they can be territorial and waspish in nature, and so might not work so well with shy or slow species, such as pipefish and small gobies.

A few marine damsels do seem to adjust to low salinity marine aquaria well, for example humbugs and sergeant majors. They are routinely kept around 1.018 by marine aquarists. Abudefduf saxatilis occurs in many different coastal waters and can adapt well to strongly brackish waters, but none of these marine damsels should be kept in water of specific gravity below 1.015. According to Damselfishes of the World by G R Allen (Mergus: Melle, Germany) there are three damselfishes that live in coastal freshwater streams and estuaries. These are Stegastes otophorus, Pomacentrus taeniometopon and Neopomacentrus taeniurus. A fourth genus Dischistodus is restricted to silty, coastal waters.

True brackish water and freshwater damsels are only rarely seen but for the brackish water aquarist they are a worthwhile challenge. For one thing, many damsels will breed in captivity in much the same way as cichlids, and any of these species is certainly worth a crack at. As with cichlids, many damsels are territorial, although the degree of aggressiveness varies. I would mix these fish with small open-water fish like livebearers or rainbows rather than with dwarf cichlids. Allen suggests Stegastes(as a genus) is best kept alone or in pairs; Neopomacentrus as small groups; and Pomacentrus alone or in pairs. This applies to the fishes behaviour toward other damselfish. Different looking fishes tend to be ignored, but cichlids might be too similar in shape and behaviour and might end up fighting with these damsels, so should be combined with care.

Stegastes otophorus

The freshwater gregory Stegastes otophorus is from the brackish and marine waters along the Caribbean and Panamanian coast. The salinity range is quoted as 3 to 31.5 ppt (i.e, about one-tenth to full strength marine). The photographs show a pretty fish, with a rounded shape rather like a sergeant major. The overall colour is initially a grey-brown, seemingly getting a deeper chocolate with age. The dorsal, anal and caudal fins have very attractive bright yellow edges. The juvenile seems to have a row of small blue spots along the back, and the yellow markings are not so bold. Maximum size is quoted at about 10 cm. In all, these seems a very desirable fish.

Pomacentrus taeniometopon

Pomacentrus taeniometopon is widespread throughout the tropical Indo-West Pacific region. It is said to occur in freshwater (occasionally), brackish lagoons, and shallow marine reefs with freshwater run-off. The juvenile is pretty, with a orange base to the caudal fin, a black and white eye-spot at the end of the dorsal, and some blue markings along the head and back. The lower parts are pale pink, darkening along the sides to a grey-blue. The adult is blue-black all over, with no other markings save some blue scribbles along the forehead. Rather like Neoglyphidiodon (Abudefduf) oxyodon. It may sometimes even be imported under that name. It differs from Neoglyphidiodon (Abudefduf) oxyodon (which is known as the black neon damsel) in not having the white, saddle-like band. Slightly smaller than the Stegastes otophorus.

Neopomacentrus taeniurus

The freshwater demoiselle Neopomacentrus taeniurus is found from East Africa to Australia. Appears to be an inhabitant of brackish rather than pure freshwater. I have seen this fish kept in a freshwater tank in a tropical fish store, and according to the dealer, it had been there for some months. The fish looked healthy enough but was very skittish. Possibly its nervous behaviour was related to the fact that it was the only specimen in the tank (with angels and Corydoras); but it may also have needed a ‘taste’ of salt to be really happy. In appearance similar to Stegastes otophorus but with more tapering fins and a more like a midwater blue chromis than a typical benthic damsel like a domino damsel. The edges of the caudal fins are also dark in colour. Salt-water Aquarium Fishes by Axelrod and Vorderwinkler (TFH: Jersey City, USA) includes this species as Pomacentrus taeniurus, an obsolete name. It gives a description largely matching Allen’s above, but adds that the upper half is purple and the lower half is olive. This would appear to be a description of a juvenile, changing colours when mature like Pomacentrus taeniometopon and many other damsels. Apparently breeds in fresh and brackish waters. 

Eel-catfish Plotosus lineatus

Warning! These fish possesses venomous fin spines. In strength the venom is more painful and dangerous than those of sea catfish (Ariidae). Large adults may deliver a sufficiently large dose to cause serious harm. If you are stung by one of these fish, seek medical attention at once.

The family Plotosidae contains so-called marine catfishes, one of which, the eel catfish Plotosus lineatus, is imported periodically, though less commonly now than in the past when fish-only marine aquaria were at the height of their popularity. As a group they are found in coastal and estuarine waters, and a few are confined to freshwater. The freshwater representatives are mainly found in Australia along with freshwater members of the sea catfish family Ariidae. There are otherwise no freshwater catfishes families native to Australia.

Eel catfish are usually imported as juveniles. The attractively striped youngsters are peaceful shoaling fishes, and should be kept in groups. As they mature they become less gregarious, and the adults are solitary. In addition it grows big, to as much as 100 cm (39 inches) in the wild. Although the juveniles are basically marine, the adults can be found in brackish waters and very rarely even in freshwater habitats. Keep the juveniles in aquaria with well filtered marine strength seawater (specific gravity over 1.018). The adults should be kept alone but can be mixed with different fish such as scats. A specific gravity of 1.010 upwards is needed for long term health. The combination of space requirements and venomous nature mean that this is really not a good aquarium fish. Read the section on adapting marine fish to brackish water to find out how to adapt marine catfish to a brackish water aquarium.

Seahorses

Note: Many species of seahorse are threatened. Aquarists have a responsibility not to add to the pressure on wild seahorse populations. you want to know more, visit Project Seahorse.

None of the traded species of seahorse are found in brackish waters. There are some species that do occur in estuaries, notably the North Atlantic species Hippocampus erectus and the endangered Cape seahorse Hippocampus capensis from South Africa, but neither of these is likely to be found in the tanks at your local tropical fish retailer.

All seahorses need a quiet tank with lots of live food. There is really no reason to keep these fish in anything other than a marine tank by themselves or with very quiet fishes such as neon gobies or mandarinfish. Kept properly, they are actually quite robust and tolerant, as well as exceptionally beautiful.

Pike-conger, Congresox talabonoides

Pike-congers are eel-like fish primarily found in tropical seas. Only a single species has appeared in the aquarium trade, Congresox talabonoides, the Indian pike-conger.

Almost nothing is known to me about this species as an aquarium fish. The specimens on sale at Wildwoods in Enfield appeared to be healthy, feeding well, and seemingly peaceable towards one another and the other fish in the tank, even after six months of maintenance.

These fish are physically rather attractive, with a streamlined, golden-green body and a beautiful fins. The head is long and tapering, with narrow jaws filled with small, needle-like teeth. Almost certainly this species will consume fish small enough to swallow whole, but it doesn’t appear to have the robust jaws typical of more opportunistic feeders or eels capable of crushing the shells of invertebrates. Wild fish are known to be nocturnal.

Maximum size in the wild is said to be 250 cm, but in all probability these fish will stay much smaller in captivity, as is usually the case with moray eels and common eels. These specimens were being sold as brackish water fish, though a moderately high salinity (SG 1.010+) is likely essential for long term success.

Bar-tailed Flathead, Platycephalus indicus

The bar-tailed flathead is not a commonly traded fish, the specimen shown here at Wildwoods in Enfield being the only specimen I've ever seen on sale. This specimen seemed to be hardy and readily took river shrimps, but otherwise nothing at all is known about the maintenance of the flag-tailed flathead under home aquarium conditions.

According to Fishbase, Platycephalus indicus is a saltwater fish that enters brackish water habitats, with juveniles bar-tailed flatheads like the specimen at Wildwoods being common in freshwater rivers. Presumably that's where this specimen was collected and how it ended up in the aquarium trade. The bar-tailed flathead is said to reach a maximum length of 100 cm, with 60 cm being more common. All flatheads are predators, and rather like anglerfish they wait for small prey to swim into range before lunging at them. They have very large mouths, so tankmates will need to be of approximately equal size.

Although this bar-tailed flathead was being sold as brackish water fish, a moderately high salinity (SG 1.010+) is likely essential for long term success. Note also the strongly flatted body shape and cryptic colouration; this species prefers to dig into the sand and remaining hidden during the day.

Bruce Hansen in Australia writes: “Neale, we eat those Flatheads here, as it is an angling species commonly caught in estuaries and beaches. They have nice mild white flesh but rather thick skin. You have to be a bit careful with the spines on the gills when handling them and can grow quite large. I have kept juveniles a few times but they grow quite quickly and have a surprisingly large mouth. Because they are angling species they have a minimum size limit for ‘taking’ which is (from memory) well over the 30 cm mark. So keeping small individuals in private aquaria is not strictly lawful here.”