We have a clarki clown in a 16 gallon qt tank. Hes been in there for quite some time since we are revamping our 65 gallon and he needs to be introduced towards the end since he has been rather aggressive towards previous fish. But to the point.. hes not using his tail fin. Hes eating normal and doesnt show any other signs of distress but the tail fin does seem slightly discolored. We dont see anything else wrong with the fin, no bumps, spots, degeneration or anything of the sort. Hell use his other fins to swim around the tank but his rear end just floats upwards, hell even sit at the top of the tank in a somewhat vertical fashion. Any advice or ideas as to what could be wrong would really be appreciated.