I have one black piranha and 4 red bellies, the reds are missing fins. Whats going on?

they live in a 55 gal. tank, they have plenty of places to hide, but recently the reds are missing more and more fins and the black wont let the reds near it, is this a gender thing or are they getting to big for there tank? please help and fast cause they are really chewing each other up!!!