If you're worried about them not getting the extra nutrition from the pellets, you can always stuff your tilapia or shrimp with the pellets. You can also use Vita Chem additive and add it to your filets/shrimp etc right before freezing it.
Either way, piranha do just fine with a shrimp/tilapia/other white fish diet.
or if your a fisherman you can feed them certain caaught fish.mine get feed mostly northern pike that i catch. there are certain things to do before doing this so if this is something your interested in ask us and we'll fill you in