Sunday, 28 August 2016

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 fluviatilisArothron 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 immaculatusArothron 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.


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 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.


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 otophorusPomacentrus 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.

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.

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