Not completely.
It's not that I don't believe the op. I don't think he is lying, but I think he is wrong. I believe, and I know this is conjecture, but I think that he just didn't give the fish a chance after he found it beat up. I believe this was caused by ignorance, laziness, and disreguard for the fish.
Also, feeding a live pet fish to a piranah seems cruel to me. Not that theres anything wrong with piranah, but dieing one bite at a time is a rough way to go out. And I'm not a person who thinks that fish shouldn't be fed live food, but feeding is no excuse for excess cruelty to a fish.