re-introducing a piranha to school

Question
Hey, my name is Dylan and i have a question on re-introducing a fish into a school. I have 5 red belly piranhas (all about 3-3.5 inch) and they seem set on making it 4, they're tank is 55 gallons and well planted with lots of room to swim. About 4 weeks ago they attacked the runt taking out one of its eye, i moved the injured fish to my hospital tank for approx 2 weeks, after his eye scared over i returned him to his school. It was only a few days after doing so before i noticed they had taken a large chunk from his head, so hes back in the hospital tank....is he going to be able to join the school again, if so how should I go about doing it? And I keep them well feed so i don't think hunger is the issue. PLEASE HELP!!!
P.S. you have a 1,200 gallon aquarium? how much do one of those cost

Answer
Hi,
Piranha are semi aggressive fish that have a tendency to bully, even to the point of death, the weakest member of the group. Because they do this, I would not recommend re introducing the injured one back into the tank, because even if they don't immediately go after him, they will sooner than later.

About the 1200 gallon tank, I built it myself, and I didn't keep track of the budget. It was less than I expected though.
Sorry I couldn't be more help.