Although im by no means an expert on the subject IME brackish fish that are forced into freshwater whether by the keeper or the breeder usually lead sad unhealthy lives and die young.A lot of the fish shops near me sell brackish fish in freshwater and i am forever seeing dead puffers,archerfish,scats,monos,shark cats and anableps in their tanks,there are some species of freshwater puffer and if puffers are what people want but they dont want to do the salt then they should stick to these.