This past weekend I bought a 10" pacu and put him in with my rhom, just to make him feel more at home. The pacu immediately tryed to shoal with the rhom, he kept pushing my rhom into the glass. After about 30 seconds of this the rhom snapped. he bolted across the tank a few times and began attacking the glass, than he turned and took the pacus tailfin completely off. After watching him hunt the pacu for a bit I went out. I awoke the next morning to find half the pacu. I don't do this all the time, but I feel it's good for the piranha to hunt big prey just like how they would in the Amazon.