I saw this in an aquarium recently, but didn’t catch the name. It’s one of the usual Amazon suspects, though:
It’s body is eel-like, not much in the way of fins, but it does have a somewhat short dorsal fin running down about the last 1/3 of its body.
I seem to recall it has a “c” in the name, but that doesn’t help much. The only one I could think of was “pacu,” and I know it wasn’t that.