They can be brackish, full salt or full fresh. BUT you can only do this at young age. I keep mine in full fresh, no salt whatsoever.
I have had this guy for a year and a half since he was 5". Eats basically EVERYTHING..smelt, krill, shrimp, pellets, as well as suchinni (him and my fei feng sahres a suzchinni daily)
Easy going, no aggressions....one of my favorite fish.
stan