I have had the opportunity to look after Red Belly piranhas on several ocassions. They are very shy fish, always swimming in a school. Having my arm in the fish tank doing what was necessary was not a problem at all. Regardless of a very good feed, the fish were very nasty to each other when they were left on their own. Usually, all I saw was just a fish head as everything else was eaten. Some individuals were bitten, usually a chunk of flesh off the back off the fish.
On one occasion when I had a fish in the net, it flicked over and in a spilt second it took a chunk of skin off my finger. I was very lucky that it was just a little fish.