Most Brackish fish can survive in fresh water, But chance's are it will not be as healthy as one that is kept in brack. Its life span will diminsh as well. It depends a lot on the type of fish. Mollys which are brack are commonly kept in FW, and generally they do alright. I had a batch of molly fry, I have 4 in FW and 2 in BW, The pair in BW are much bigger then there FW siblings so it sorta of proves my point even more.