This page will help you better understand what piranhas eat-- in the wild as well as in the aquarium.
In nature, piranhas use "gorilla-warfare" on their unsuspecting victims. Piranhas by nature are scavengers, as they routinely "weed out" sick and dying fish or animals. They usually lie among the cover of plants and rocks until their would-be victim passes within striking distance. Although this is not true of the fin and scale eating piranhas, we will focus our discussion on the fish eating piranhas due to the fact that these are the piranhas most likely to be found in the local pet store.
Young piranhas are someone more aggressive in their search for food, but this aggressiveness generally declines with age. As piranhas mature, they rely almost exclusively on the "ambush" attack unless driven by severe hunger. The piranha has an acute "sniffing" ability, and can smell blood in the water much like a shark!
This is a young red-bellied piranha. (serraslamus nattereri) A bite from this juvenile piranha wouldn't require more than a kiss from mom, but this same piranha in 3 months could take off an entire finger!
O.K.----now that we've covered feeding the piranha with your own flesh....... let's get back to what you SHOULD feed the piranha. :)
Young piranha, (those with the silvery color pattern similar to the picture), should be fed a mixed diet of live "feeder" fish, flake or pellet food, and live worms. Your local pet store carries all of these and can help you with choosing.
This is a picture of the same piranha as the above photo in a mature state.
This mature piranha should be fed the same thing as the juvenile, but in larger portions. Consider buying quite large feeder fish, as some piranhas get quit lazy in ther old age and just won’t use the energy to chase down the smaller ones. Also, feed your piranhas some beef heart. It is great for their growth, and much cheaper than feeder fish, but continues to offer a mixed diet. Strips of beef heart can be cut up and individually frozen. Just pull out a strip and cut chunks off of it. Keep in in the refrigerator for feeding the next day. It’s as simple as that!