Monos, family Monodactylidae

Monos, family Monodactylidae

Three species of mono are traded, Monodactlyus argenteus, Monodactylus kottelati, and Monodactylus sebae. The first two species are not distinguished in the hobby and are simply called common monos or Malayan angels. The third species is known as the West African mono and is easily distinguished.

All three species require the same basic care. They are not fussy about salinity, though adults at least fare best in moderate to high-end systems (SG 1.010 upwards). High levels of carbonate hardness are essential, and the use of calcareous material, such as crushed coral, in the filter to provide this is recommended. A protein skimmer will help maintain good water quality. Monos are primarily plankton and detritus feeders in the wild, and in aquaria will eat most foods including flake and pellets. They are lightning-fast animals, and need plenty of swimming room; Monodactlyus argenteus and Monodactylus sebae at least should not be kept in a tank less than 120 cm long.

Monos are essentially quite robust, particularly once settled in. It is easier to introduce juveniles into an aquarium than adults. Adults seem to be sensitive to shock when moved, turning black instead of their normal silver. Left alone they will recover eventually. In the wild, juvenile monos are reported to live singly or in small groups, forming large groups only as they mature and move into open water. Under aquarium conditions the adults tend to be highly territorial and sometimes aggressive towards other members of their species if overcrowded. Odd numbers of specimens work best if you want to keep them in groups. Different mono species mix well, and they also mix well with scats, and keeping them in mixed species groups seems to moderate their aggression somewhat.

Not much is known about the breeding behaviour of these fishes. They appear to spawn in freshwater, and the few reports of breeding in home aquaria have occurred when these fish were being maintained in freshwater conditions. Since these fish are more disease-prone when kept in freshwater tanks, maintaining them under such conditions is not recommended, and attempts to breed these fish are best left to expert fishkeepers able to move them between fresh and brackish water tanks as required.

Distinguishing the common species of Monodactylus

Monodactylus argenteus is the most widely traded species. This fish is found from South Africa east to Australia. It has a circular body bearing triangular dorsal and anal fins of similar size. The dorsal fin is marked with yellow (though this may fade to orange as the fish mature). In captivity this species can be expected to reach a length of 15 cm (6 inches) though it does get much bigger in the wild.
Very similar to the the common mono is Monodactylus kottelati. It has a similar shape, but the dorsal fin is not as tall as the anal fin is deep. This is the most reliable way to distinguish the two species. In addition, the dorsal fin on this species tends to be orange, whereas on the common mono this fin is yellow. It is a small species said to reach no more than 8 cm in the wild, though maximum size in captivity is uncertain given the problems distinguishing it from M. argenteus.
Restricted to Africa is the fingerfish Monodactylus sebae which is much more deep (and angelfish like). It lacks the yellow of the Malayan angel, but the dark grey or black bands running from the dorsal to the anal fin are much bolder. This fish is much deeper than it is long, and needs plenty of depth. Don’t keep adult fingerfish in aquaria less than 60 cm (24 inches) deep.

Books

Compared with other branches of the fishkeeping hobby, there are very few books specifically on brackish water fishkeeping. My own book covers practically all the fish species currently traded and includes chapters by world-experts on topics such as mudskippers, gobies, and Australian brackish water fishes.

Two others of note are Brackish Aquaria by Michael Gos and Aqualog’s Brackish Water Fishes by Frank Schäfer. The former has been out of print for many years and is difficult to find. Of the two, it is perhaps the simplest and easiest to read, but even so, it contains a fair amount that is at best debatable, and quite probably erroneous. The Aqualog book is excellent but concentrates on species requiring moderate to high salinity, and thus ignores many of the cichlids, livebearers, and other fishes that do well in low-salinity aquaria.

• Brackish Aquaria, by Michael Gos, published by TFH, 1979, ISBN 0-876665199
• Brackish Water Fishes, by Frank Schäfer, published by Aqualog Verlag, 2005, ISBN 3-9360-2782-X
• Brackish Water Fishes, edited by Neale Monks, published by TFH, 2006, ISBN 0-7938-0564-3

Other books that cover brackish water species or aquaria as well include the following:

• Aquarium Atlas (vol. 1), by Rüdiger Riehl & Hans Baensch, published by Mergus, 1987, ISBN 3-8824-4050-3 (This book has been revised numerous times, and additional volumes are also available)
• Complete Aquarium, by Peter Scott, published by Dorling Kindersley, 1995, ISBN 0-7894-0013-8
• Pufferfish, by Chris Ralph, published by Interpet, 2003, ISBN 1-8605-4233-6
• The puffers of fresh and brackish waters, by Klaus Ebert, published by Aqualog Verlag, 2001, ISBN 3-9317-0260-X

Brackish-intolerant Aquarium Fish

Freshwater Fish

Obviously the best fish for a brackish water aquarium are fish that naturally come from brackish water, but it often happens that an aquarist wants to mix some brackish water species into a regular community tank (or vice versa). For the most part this is a bad idea -- most of the commonly traded community fish are not salt-tolerant and will not do well in brackish water. With a very few exceptions, tetras, barbs, danios, rasboras, gouramis, and loaches should not be kept in the brackish water aquarium.

Catfish are a mixed bag in terms of salt-tolerance. There are of course several families with species that enter the sea, including the Aspredinidae, Ariidae, Bagridae, Pangasiidae and Plotosiidae. But the two families most frequently kept by aquarists are the Callicthyidae and the Loricariidae, and these families are made up almost entirely salt-intolerant catfishes.Corydoras species as well as most of the plecs and bristlenose catfish should not be kept in the brackish water aquarium. Exceptional members of these two families include some species of Hypostomus such as Hypostomus plecostomus known to be slightly salt-tolerant (SG to 1.005) and the brown hoplo Hoplosternum littorale, a species commonly found in brackish water in the wild and easily maintained at SG 1.005 or even slightly higher.

Some cichlids are quite salt tolerant, of which more is said elsewhere in this FAQ, but the Rift Valley cichlids apparently react badly to brackish water, succumbing to “Malawi Bloat”, a potentially fatal dropsy-like condition. Cichlids from soft water environments, including angelfish, are equally intolerant of salt.

Livebearers and killifish from hard, alkaline waters often do fine in slightly brackish water. So while things like platies, goodeids and pupfish might not come from brackish waters in the wild, they can be expected to adapt successfully to a specific gravity as high as SG 1.005.

Marine Fish

Some traded marine fish will tolerate low salinity conditions for extended periods, even indefinitely, but in general this should not be assumed. If you want to add marine species to your brackish aquarium, do your research carefully and make absolutely sure the species being offered in the shop is what you think it is.

Commonly Traded Species of Brackish Water Fish

Although brackish water aquaria are less frequently created than either fresh or marine ones, brackish water fish are far from uncommon and in fact include some of the hobby’s staples. A few of the commonest and most desirable inmates for the general brackish water aquarium are:

• Sailfin and other mollies
• Monos
• Scats
• Archerfish
• Pufferfish
• Cichlids

Less frequently seen, but very desirable for the aquarist eager to try something a bit different, are the predators of the coastal swamps, estuaries and mangroves, such as:

• Gar
• Shark catfish
• Sleeper gobies
• Spiny eels

And a few oddballs quite unlike anything else:

• Anableps
• Mudskippers

Even the aquarist with only minimal space can enjoy the fishes which are happy in small tanks (24 inches in length or less):

• Killifish
• Gobies
• Glassfish
• Pipefish

All these fishes and more are covered in later parts of the FAQ.

Basic Equipment

The main difference between a freshwater aquarium and a brackish water aquarium is water chemistry. Brackish water needs to have a high level of general and carbonate hardness, a pH of between 7.5 and 8.5, and a salinity between 20-50% that of normal seawater (SG 1.003-1.012) depending on the species being kept.

The following items are essential:

• A glass or plastic aquarium (metal corrodes quickly in contact with brackish water)
• Marine salt mix (not aquarium “tonic” salt)
• Plastic buckets for mixing up brackish water
• Hydrometer or refractometer for measuring specific gravity

Some useful but optional components include:

• Silica sand substrate for burrowing fish
• Salt-tolerant live plants
• Calcaerous filter media such as crushed oyster for buffering the pH
• Protein skimmer (works from SG 1.010 upwards)

Where Do Brackish Water Habitats Exist?

The environments from which brackish water fishes are collected are diverse. Estuaries are the best known. An estuary is the part of a river where it meets the sea. Typically, estuarine waters are slow and sluggish, and often very silty and fertile. As a result they are not always as attractive to look at as the clear waters of a mountain stream, but they are tremendously productive. One characteristic of estuarine ecosystems, and brackish water habitats in general, is that while productivity (the amount of food available) is high, diversity (the number of species) can be quite low compared with rivers or the sea. This apparent contradiction is because relatively few fish and invertebrates can tolerate the fluctuations in salinity. On the other hand, those animals that can live there do so in enormous numbers. Gobies, flatfishes and catfish are characteristic fishes of estuaries, ranging from the fresher waters upstream right down to the sea. A few freshwater fishes may occur in the least saline parts of the estuary (for example garpike, ropefishes and cichlids). Many marine fishes, especially as juveniles, inhabit the saltier part of the estuary (such as sea bass, flatfishes, tarpon and herring).

Another important brackish water habitat is the mangal (or mangrove swamp). Mangals are characteristic of the tropics. Some mangals develop in estuaries, while others fringe islands. Mangrove plants are some of the most remarkable plants to be found anywhere in the world. They form dense forests which support a tremendous variety of animals both above and below the surface of the water. In the canopy, monkeys, snakes and a huge variety of birds are to be found; while the aquatic roots support oysters and barnacles, as well as many fish, snails and crustaceans which live in and around the thick knots of roots and stems. Mangals have provided many good fish for the aquarist, such as mudskippers and archer fishes. The mud that collects within the mangrove roots is a prime habitat for fiddler crabs, various clams, and snails.

The temperate equivalent of the mangal is often said to be the salt marsh. These are tidal habitats, periodically covered by the sea. Salt marshes are also important habitats, especially as the feeding grounds for seabirds and fishes. Instead of trees, the major plants are grass-like. Gobies and sticklebacks are especially common in salt marshes, which are quite easy to explore and a good place to collect native fish and invertebrates for the aquarium. Unfortunately many salt marshes are threatened by agricultural or commercial development.

Common to both the tropical and temperate zones, seagrass meadows are found in shallow water. Seagrasses are true plants and not algae, but have proved to be difficult to keep in aquaria. There are many fishes and invertebrates found in sea grass meadows, perhaps the best known is the seahorse. The salinity of a seagrass meadow is rarely much below that of full strength seawater, but seagrasses do occur in lagoons and estuaries where the water is brackish.

What is Brackish Water?

Brackish water is water which contains more sea salts than freshwater but less than the open sea. Moreover, brackish water environments are also fluctuating environments. The salinity is variable depending on the tide, the amount of freshwater entering from rivers or as rain, and the rate of evaporation. As a result many brackish water fishes are tolerant of changes in salinity, and in fact many positively benefit from similar periodic changes in aquaria.

On the other hand, filter bacteria will not tolerate big changes in salinity. So in the aquarium it is normal just to make changes a point or two up or down the specific gravity scale, i.e., from SG 1.005 to 1.006, or down from 1.012 to 1.010.