I have one Pygmy Cory that seems to have lost its tail fin. It can still swim but does not seem too happy. I do have a couple of Silver Tipped Tetras that are aggressive but they mainly chase. I have never seen them fin nip. The only other reasons I can see for the loss would be either some kind of fin rot or else the grim possibility that I might have cut it when doing some trimming in the back of the tank, which is dense and a place the Cories like to hang out. Can tail fins grow back?