Well I recently stocked my tank. I got 8 cardinal tetras, 4 Serpae tetras(were already in tank), 5 black phantom tetras, and a bristlenose. Along with a stray long finned serpae type thing. It wouldnt stay out of the net so she just gave it to me. Anyways I'd say about half of the fish have nipped tails now. I've tried watching the tank closely to see whos doing it, but its very hard to tell. I havent seen anything go after the cardinal tetras yet. The serpae tetras and the long finned one seem to go at it quite a bit, but it seems like more chasing then actual attacking. They get next to each other and swim at a wierd angle, and sorta slap each other with their tails. I dont know if thats normal, or maybe some sort of breeding habbit. Well anyways, if anyway has an idea on who the culprit may be please let me know so I can remove them. I dont want to lose any fish!