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.