I had the same problem with a jewel cichlid. It was when I first got my tank and I didnt know any better, so I put the jewel with some haps and a yellow lab. After a week everyone was beat up and I couldnt figure out who was doing it. I finally caught the jewel cichlid in the act one day. I never would have guessed it though if I didnt actually see it because he always hid in the back and seemed too timid to be agressive. One of my fish was missing a huge chunk of his tail all the way down to the body, it grew back completely within a couple weeks.