Well over 15 years i gave them all kind of stuff. I feed fish once a while really any type i get cheap like goldfish. I also put some random stuff in, that i catched in rivers or lakes. We have a dead channel here, wich is full of fish like to millions at the sides when it gets warmer so it wont really hurt nature i guess, to take a few. The names are: Silverlings, red feather, redeyes, graskarpfen, forelle, stichling. I live in germany tho if you wonder bout that names, not sure if i got them all right translated. I also have a breeding store for outdoor lake- and riverfish near my house where i got alot fish for good prices. But well they also ate frogs , mice, craps ,catfish, sunbarsh yet. And well frogs or mice is to much of a mess and well it can end up pretty cruel if them not hungry so i dont do that anymore. Also i never give them anything alive if them not hungry. I make sure to not feed them at least 3 days and then put it in so it mostly takes like 5-10seconds untill the animal is complete eaten.
Edit: Oh that reminds me i also raised some breeds of them yet. The small redbellys i fed with living maggets from a fishing store. That looks hell cute when them like fingernail size with big eyes. At that time they really eat more then they possible can. Its one of the most funny things i ever seen. They end up laying on the ground with half a magget sticking out the mouth digesting it yet but they wont let go of it. Also the tiny larvas i fed with brine shrimps, its the only thing they eat. Most piranha keeper dont know that so the larvas die once they hatch and used the yolt sack if the parents didnt eat them. I put them in a seperate tank and raised them there.