Freshwater flatfish, order Pleuronectiformes
Freshwater soles and flounders are commonly offered to aquarists as novelty fish, but unfortunately they are relatively difficult fish to look after well. All are shy, nocturnal, and exhibit a distinct preference for live foods. None can compete with more active or aggressive fish, such as catfish or cichlids. Consequently many, if not most, of the freshwater soles sold to aquarists end up starving to death. The name ‘freshwater sole’ is also somewhat misleading, given that many have a distinct preference for brackish conditions over plain fresh water.
Because it is so difficult to identify these fish to species level, deciding on the optimal salinity for these fish is tricky. While a few species, such as Brachirus harmandi, Synaptura salinarum, and Cynoglossus waandersii, complete their entire life cycle in freshwater, most of the species traded as freshwater soles in fact move freely between fresh and brackish (and even fully marine) waters. Adult hogchoker soles, for example, can be found far out to sea as well as hundreds of miles up rivers. Nonetheless, many aquarists have found that they do best in brackish rather than fresh water. Therefore unless you can positively identify your ‘freshwater sole’ as a truly freshwater species, it is probably safest to keep it in slightly brackish water (with a specific gravity of around 1.005). The truly freshwater species can easily tolerate a specific gravity this low, so even if you happen to misidentify a freshwater species as a brackish water one, no harm will be done.
Freshwater soles also seem to be intolerant of low oxygen levels and will swim to the surface and ‘gasp’ if the water is not adequately aerated.
All freshwater flatfish require a aquarium with plenty of open sand or fine gravel into which they can burrow. Because these fish can alter their coloration so that they can blend in with the substrate, brightly coloured substrates (such as blue or red artificial gravels) must be avoided. River sand, smooth silver sand, and pea gravel are good choices, but remember that sandy substrates cannot be used with undergravel filtration systems. Ornaments, rocks, and plants can be used in tanks containing freshwater soles, but these should be kept to the edges or back of the tank so that plenty of open sand or gravel remains.
These fish are essentially hardy and do not seem to be as prone to diseases like whitespot as many other aquarium fish. Neither do they seem bothered by high levels of nitrite or ammonium, at least over the short term. I have used Brachirus sp. to run in a brand new aquarium without any obvious problems. However, these fish are difficult to feed because they are so nocturnal, and even when they do swim about in the day, they do not seem to feed. Frozen bloodworms and mysis shrimps, small pieces of prawn and squid, and live foods such as earthworms and river shrimps, are all taken by these fish, but it is essential to offer these at night once the lights are out and the other fish in the tank cannot feed.
The most commonly traded species are:
Family Achiridae (American soles)
Family Cynoglossidae (tongue soles)
Family Soleidae (true soles)
Telling these families apart is generally not difficult: the Achiridae usually have rhomboid bodies with small heads but large, paddle-like tails. The Cynoglossidae are very elongate and leaf-like, with small, tapering tail fins but a large, shovel-shaped head. Finally, the Soleidae have a leaf-like shape similar to the Cynoglossidae though not always so elongate, and the head is small and rounded. The FAO fisheries guide to Sri Lanka includes some helpful diagrams for distinguishing typical examples of the Cynoglossidae from the Soleidae.
Besides the species listed above, other species may be imported from time to time, but identifying any of them is tricky because there are several lookalike species and all are able to change their colouration and markings to some degree.