tounge fish

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Are you talking about Euryglossa pan? I had a few of these guys in freshwater for a long while, used to feed them on live and frozen bloodworm. They wouldn't take anything else, if you read about them, they are from around the far east, they should be kept in brackish water and in a species tank. They don't compete very well with other fish.
The Archius species are more hardy and have certain members that live most of their lives in freshwater, like the Peruvian flounder.
Hope you find some more info.
 
There are lots of tongue fish, 136 species, about 1/2 are strictly marine, most of the rest are brackish, a couple dozen move freely between marine and fresh, 5 are true freshwater. I am pretty sure that those five are all Cynoglossus genus and are the
waandersii 3 1/2"
heterolepis 9"
felomanni 10"
trigrammus 12"
and (pictured)
microlepis 13"

Some of the "all waters" ones like the semilaevis get pretty big (23")
they are all happy to eat small fish, inverts and meaty foods and can learn to eat sinking pellets and chopped clams, they like fime gravel or sand, don't keep them with plecos as the plecos suck on them (lost an 8-9" microlepis that way)

There are several true sole that look like tongue soles and are brackish as well.

cynoglossus-microlepis.jpg
 
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