I started with 4 piranhas, and one was measurably smaller than the others, he also had one eye, hence i called him willy. Willy was a good fish, he often started the feeding frenzy, but then had to back away as the other p's took over. I usually feed my p's daily and keep goldfish and guppies in there so they don't nip at each other's tails. I went about 4 days without feeding my p's and thats when i noticed that willy no longer had a body or tail, just a head. You gotta keep a few guppies in there so the p's can chase them around cuz otherwise they start chasing the weak one.