My cousin has a scat as his avatar. That fish was strictly fresh for its entire life and it was huge, healthy and happy. Search user drb.
I am a firm believer that a naturally occuring brackish fish will be absolutly fine in fresh if it was born and raised in fresh only. I will further say, you may cause undue stress to a fish by trying to put it in a brackish tank when it has been fresh its whole life, and same goes for a brackish to fresh switch.