You do not want to feed your piranhas beef heart everyday if you want the to stay healthy. They need a varity of stuff in their diet. In the wild, piranha's diet consists of over 90% fish ...
Mix it up a bit. Give them krill, fish (cod, catfish etc.), shrimp, etc. I even have my piranhas eating "veg-all"
IMO you should only feed them beef heart once a month or so as a treat..