I got up yesterday to see that one of my pepper cory cats was floating at the surface. Thinking he'd passed away, I opened the lid to get him out and he panicked a bit, then went back to the bottom. Also saw that he'd lost most of his tail, though his other fins seem to be in good shape. The nitrates levels were quite high so I did a large water change and added some Melafix. Since then I've noticed him hanging out at the surface several more times, usually on his side with belly against the glass. Could this behavior somehow be due to his tail or is there another problem I haven't noticed?