It is tough to tell why with out more info. If you rule out tank mates then do a 50% water change. Check your Ph to make sure its not too high/low and the rest of the parameters A. N. N., hardness ect. GTs accept a pretty wide range of water conditions 6.5 to 7.5Ph and soft to hard just make sure everything else is good and keep up with the weekly water changes, the fins should heal. You could add some Novaqua to the water at each water change also.