There have been numerous threads on freshwater soles on these forums... do a search and you'll find quite a bit.
If you have access to this month's TFH magazine, I have an article about them in there. Only one book has a whole chapter on them, the Aqualog brackish book. There's some more stuff in the
Brackish FAQ.
Basically, as AMS states, many species are brackish to marine, particularly the species from the Americas. Of the species exported from Asia, about half a true freshwater fish or need only slightly brackish water, while the rest need high end brackish to marine. True freshwater species cannot be kept in high-end brackish or marine.
Identifying soles is a pain, and very difficult. You need to spend some quality time on Fishbase searching through the different species of Achirus, Trinectes, Brachirus, Synaptura, Cynoglossus, etc., and even then, you can make mistakes. Because some species need salt/brackish water and some freshwater, identifying them is important.
All soles need the same basic things: a sandy substrate and regular feedings of live bloodworms AT NIGHT for the first few weeks after import. No not mix with catfish or loaches, or any other fish likely to steal food from them. I have kept them with puffers, but it's chancy, because a curious puffer might decide the sole's eye is a tasty morsel. Ouch. Better tankmates are mid to upper level fish, like halfbeaks, guppies, danios, hatchetfish, tetras, etc. (depending on whether the sole is fresh/salt water, of course).
Cheers,
Neale
