I would loo into getting some coppersafe or maracyn. It looks like your GT may have some fin rot going on which is very bad. In fact, I would bet money on it. See in some of the pictures how the end of the fin (before the wirey section) has a whiteish tip? The whitish tip is a key sign in fin rot. If I am right then just keeping up water changes won't fix it. It is a bacterial infection that must be treated and the infection cured before it reaches the base of the tail. It will keep disintegrating leaving only thee wirey ends of the fins, and eventually those will fall off and your fish will look stumpy and most likely die. A fish cannot live without a tail fin. I could be wrong but thats what my female JD had after getting beat up pretty bad by my male. If the rot reaches the base of the tail then the tail will most likely not grow back. Good LUCK!