I had a fancy gold that lost it's tail in the filter. It was staying on the bottom most of the time but came back. If you leave the fish that lost it's tail in that tank it will most likely die, but it has a chance if you move it to another tank. About your first question, I'm really intrested in what the answer is, I always do research to find out if the fish I want/have(in seperate tanks) will be compatible, but Ive never really found out why some fish nip and others don't.