Flatfish are among the most bizarre of all bony fish. Although superficially similar to the stingrays, their flattened appearance comes about in a totally different way. Stingrays are flattened from top to bottom, and so while they do have an unusual shape, the layout of the fins, eyes, and mouth is essentially the same as that of any other fish. The eyes are at the top, the fins on the side, and the mouth underneath. Flatfish, in contrast, are flattened from left to right, much like a discus or angelfish, except that instead of swimming in the normal way, they swim on one side. Some swim on the left side, and some on the right, but either way the anatomy of fish becomes extensively modified to accommodate this. If you look at a flatfish that swims with its left side downwards, like the hogchoker sole Achirus fasciatus, you'll see it actually has both eyes on the right hand side of the fish (which in practical terms is now the upper surface of the fish), the mouth is on one side of the fish instead of underneath, and the tail fin doesn't beat from side to side as with other fish, but up and down. From our perspective looking down onto the fish, we can also see that the fins along the edges of the body are the anal and dorsal fins, and the pectoral fins are in fact positioned where we would expect the anal and dorsal fins to be!
Unsurprisingly perhaps, the flatfish group -- the order Pleuronectiformes -- is among the most recent to evolve and is generally considered to be an extremely advanced member of the bony fish group. There are hundreds of species of flatfish, mostly in the sea, making them also one of the most successful groups of as well. While most are of moderate size, typically around 30 cm (12 inches) in length, there are some real giants, including the halibut, which is able to grow to lengths of well over 3 metres (10 feet). In their favoured cold, deepwater habitats, fully grown halibut must be predators to be reckoned with, but sadly overfishing has made such monsters relatively rare and little is known about their biology.
Most flatfish are cryptically coloured, that is, camouflaged, and have a pale or mottled coloration that allows them to blend in with their surroundings. Several species are able to change their colour to match their substrate even more closely, and most scatter a thin layer on sand on top of themselves when they are resting to finish off the illusion. Concealment allows them to avoid their predators of course, but flatfish are also hunters themselves, and camouflage allows them to stalk their prey very effectively.
Though unquestionably fascinating animals, for various reasons freshwater flatfish have never been very popular with aquarists, even though several species are regularly traded. Why is this? More than anything else it has to be that these fish are not particularly active and invariably nocturnal, making them rather dull compared with things like puffers or cichlids. They are also extremely good at hiding, and tend to spend most of the day buried under the sand. Couple this with their pickiness at feeding time (they don't care much for flake) and you have a fish that many consider a waste of filtration capacity.
That's a shame, because these are actually rather hardy, interesting animals that can work well in a community tank, and once settled in they become much less shy. Though they are unlikely to become as bold or inquisitive as, say, a pufferfish, an acclimated and comfortable flatfish will swim about by day and provide its keeper with a great deal of satisfaction. To see a large specimen stuck to the front of the glass is a real treat, allowing the aquarist the chance to look closely at one of the most bizarre body morphologies in the entire animal kingdom. Moreover, unlike the stingrays, these are not expensive, high-maintenance animals, and demand only a few basic things to do well: clean water, a soft substrate, plenty of oxygen, and nightly feedings of bloodworms, shrimps, and other meaty foods.
Although quite commonly called freshwater flounders, all of the fresh and brackish water flatfish that are traded as aquarium fish are in fact soles, members of the families Achiridae, Cynoglossidae, and Soleidae. Only a few species turn up regularly, and these are listed below, though others do turn up from time to time. Generally they make good choices for a community tank with other quiet, but not too small, species of fish. The main differences between these species is whether or not they require fresh or brackish water; otherwise, all behave in much the same way and enjoy the same sorts of food.
Order Pleuronectiformes (flounders, soles, and other flatfish)
Family Achiridae (American soles, hogchokers)
Also known as the hogchoker sole, many older aquarium books refer to this fish by an obsolete Latin name, Achirus fasciatus. One of the few North American tropical fish, this species can be found in fresh, brackish, and salt water from Florida along the Gulf Coast through to Texas. Juvenile hogchokers are generally a sandy colour with dark brown or grey peppering, but adults often sport narrow transverse stripes that make them quite distinct from any of the other common freshwater flatfish. Another key difference is their shape: these fish are close to circular, and the anal and dorsal fins end well clear of the tail fin; in contrast Brachirus and Synaptura species tend to be much more tongue-shaped, and the anal and dorsal fins more or less merge with the tail fin.
An adaptable species, this species does best in a slightly brackish aquarium along with fish like mollies and halfbeaks that stay in the upper regions of the aquarium. Although by far the most commonly traded freshwater flatfish in the US, many dealers get these fish in when they are very small. Such fish are tricky to feed because they do not compete well with more active fish, and so are best raised in a tank by themselves. A peculiarity of this species is its taste for algae. Although not a major part of its diet, it does seem to like some greens, so a mature aquarium with at least some green algae on the stones or glass is probably ideal. These fish can grow to up to 15 cm (6 inches) in length, although this is uncommon in aquaria.
Family Cynoglossidae (dog-tongue soles)
The dog-tongue soles are very elongate soles with bodies that taper almost to a point, the tail fin itself being barely discernable. This gives these fish an extremely leaf-like shape. They could potentially be confused with true soles like Brachirus pan that also have a tapering body, but the tail fins on those fish are more paddle-like and much more obvious. Dog-tongue soles also have rather long, almost triangular heads giving them a very distinctive appearance.
A few species of Cynoglossus are imported very occasionally, and none has had any lasting impact on the hobby. Some are truly freshwater fish, but many are brackish water species, and being difficult to tell apart choosing the ideal water conditions for these fish is tricky. It is probably safest to keep these fish in slightly brackish water. Otherwise, their maintenance is comparable to the other soles, with a preference for soft, sandy substrates and a variety of meaty live and frozen foods offered at night. Most of the species offered to aquarists reach lengths of around 20 cm (8 inches) when fully grown.
Family Soleidae (true soles)
Sometimes called the 'pan sole' or referred to by an obsolete Latin name, Euryglossa pan, grows to lengths of around 10 cm (4 inches) and is the most elongate of all the freshwater flatfish you are likely to come across. It has a distinctly tongue-like shape, being three or four times longer than it is wide, with the widest point being not far behind the head. The fins along the edges of the body merge almost imperceptibly into the tail fin to form a blunt point. The basic pattern on the top of the body is a light grey or brown with darker spots and blotches, but this varies depending on the mood of the fish.
Brachirus pan is a truly euryhaline fish, and will adapt to fresh, brackish, or marine conditions, but in captivity it is most easily maintained in a community tank with slightly brackish water along with medium sized livebearers, glassfish, and gobies. Being relatively small and inoffensive, this fish can be trusted with all but the smallest companions. It also gets along well with others of its own kind, though it does not seem to school with them or seek out their company. As with all the freshwater flatfish, there are no records of captive breeding. Brachirus pan comes from South Asia, in particular India and Bangladesh, neither of which are major exporters of aquarium fish. As a result, this is one of the less commonly encountered freshwater flatfish.
This species is very frequently mixed up with the Brachirus pan, and the two fish are indeed very similar in form and habit. However, this fish does get rather larger, up to 20 cm (8 inches) in length, and being a predatory animal, poses a more significant threat to small fish such as livebearer fry or very small killifish. Brachirus panoides is less elongate that Brachirus pan, being about twice as long as it is wide, and the widest point of the fish is about halfway down the length of the body. In other words, where Brachirus pan is tongue-shaped, Brachirus panoides is much more oval. The top of the fish is essentially off-white or grey in colour with darker grey marbling and spots, varying somewhat depending on the surroundings and mood of the fish.
Another tolerant species, this flatfish occurs in both fresh and brackish waters, and will do well in either hard, alkaline freshwater or slightly brackish water. Brachirus panoides comes from South East Asia, in particular Thailand and Indonesia, and so is quite commonly included in shipments of fish from that part of the world. But thanks to the similarity between the name of this fish and Brachirus pan, it is quite likely to be exported under the wrong name.
This widely distributed freshwater flatfish is most likely to turn up in shipments of fish from Australia, although it is known from lots of other islands in the Pacific as well. Sometimes referred to as the saltpan sole, this species is an inhabitant of freshwater streams and rivers. It is very adaptable, and will do well in most aquaria with neutral to slightly alkaline water conditions. Adding a little salt may help in soft water areas, but these are not really brackish water fish so there is no need to raise the specific gravity above 1.005.
Synaptura salinarum is a relatively rounded fish compared with the species of Brachirus already mentioned, but unlike Trinectes maculatus, the caudal fin is not obviously distinct from the anal and dorsal fins, and seems to form an extension to them instead. It is also much less circular than Trinectes maculatus, and while not quite so elongate as some Brachirus, is still a relatively tongue-like in shape. Although variable in coloration and pattern, this fish tends to be mottled grey or brown and covered with irregular spots and blotches. It grows to about 15 cm (6 inches) in length, and while it should not be trusted with very small fish, is otherwise completely peaceful.
The flatfish aquarium
Except for slight differences in their preference for salt, all these flatfish need a similar sort of aquarium. Depth is largely unimportant, but an open, sandy bottom is. Rocks, plants, and other dÃ©cor can be used, but it is critical to leave a large area that is uncluttered and available for these fish to burrow into. Even when settled in, they will spend most of the day in the sand, virtually invisible. A soft substrate is absolutely essential for the well being of these fish, and really the only way to get to see them behaving naturally. Silica (or silver) sand is ideal, but beach sand can also be used with success. The main problem with sand is that it is incompatible with undergravel filters, so an aquarium rigged for flatfish will need to use an alternative type of filtration. Food getting lost in the sand and causing anaerobic decay isn't much of a problem because the constant digging by these fish should keep the grains from getting too compacted. Even so, occasionally stirring the sand and scooping up any waste is probably a good idea.
While these fish are quite hardy with regard to ammonium and nitrite levels, making them tolerant of occasional lapses in water management, they are sensitive to low levels of oxygen. This is best handled in two ways. Firstly, do not keep the aquarium too warm, and secondly, use additional aeration or arrange the filter so that the outflow of water creates plenty of turbulence. The better the water is oxygenated, the happier this fish will be. Beyond that, these are remarkably robust fish. Some aquarists have noted that they are among the last fishes to get infected with external parasites like whitespot, possibly due to their rather slimy, slippery skins, but whatever the reason, so long as these fish are fed well, they should grace your aquarium for years.
As noted in the species entries, most 'freshwater soles' in fact prefer slightly brackish water, a specific gravity of around 1.005 being a good starting point. Of the commonly traded species, only Synaptura salinarum is believed to be entirely confined to freshwater. Higher salinities will not harm the other species at all, and in fact Trinectes maculatus and Brachirus pan will both do well in anything up to full strength sea water. Indeed, with some species, increasing the salinity as the fish ages may be essential, particularly if you find that your sole is not eating or appears to be unhealthy or restless. 'Mystery' species in your local tropical fish store that you cannot identify are best assumed to be brackish, rather than freshwater, fish; keeping truly freshwater species in slightly brackish water will do them no harm, and the brackish water species will obviously be much happier.
Feeding is an important topic to consider before buying a freshwater flatfish. Unlike most community fish, they will not take food during the daytime, certainly not at first. These are essentially nocturnal animals, and you will need to work around that fact if you are to keep them successfully. Food will need to be offered at night, preferably with the lights out, and it is critical not to have other nocturnal fish in the tank with the flatfish that might eat all the food before the flatfish gets a bite. In other words, these fish shouldn't really be kept with catfish or loaches. Day-active fishes, on the other hand, like gobies and livebearers, make ideal companions. At first you will probably need to feed your flatfish live or frozen foods such as bloodworms, mysids, or brine shrimps, but once settled in some specimens will also take catfish pellets and chunks of prawn or white fish. As noted earlier, some will also eat some algae as well; however, in no sense are these fish scavengers though, and you cannot expect them to 'clean up' behind the other fish.
Enjoying your freshwater flatfish
Freshwater flatfish are among the niftiest inmates for the community tank because they are so completely different to anything else the aquarist keeps. Even when resting in the sand, they can be fascinating to watch. Here are some things to look out for:
Once settled in, you'll get to see your flatfish move about a bit more. This is very rewarding. When the fish swims, it rarely uses its tail fin, and instead undulates the anal and dorsal fin to scull itself along slowly. As it does so, it barely lifts itself off the ground, and frequently leaves behind distinctive trails in the sand. Only when frightened will it sprint forward using its tail fin for propulsion; though hardly elegant, flatfish are blindingly fast over short distances. Flatfish occasionally settle on the front glass of the aquarium, which allows the aquarist to see the 'blind side' of the fish. Invariably white or pink in colour, the amazing thing about this view is that the skin on this part of the animal is so thin it is easy to see all the muscle blocks on the inside of the fish. In short, it looks an awful lot like a skinned fillet of plaice!
Freshwater flatfish are fascinating and hardy aquarium inhabitants that have not received anything like the attention they deserve. While they may not be the perfect fish for the beginner, or someone who simply wants bright colours or scintillating behaviour, they do have a lot to offer the experienced aquarist in search of something unusual. Few fish deliver so much weirdness with so few demands. Never mind the bleeding heart (tetras) of your aquarium -- isn't it time you gave it some sole?