Be consistent and don't give up... I have had greater success in starting them off with either chopped shrimp or salmon fillet. If they don't eat one, they will usually eat the other. I basically chop them up into bite-sized pieces and leave them there overnight if I have to. They are usually gone by the morning. If not, I remove it and wait a couple of days before doing it again. I've never heard of a healthy piranha starving to death. After I get them eating these prepared foods, I go on to add other fish fillet, beefheart and nightcrawlers for variety and they never see another feeder fish.
So far, I have gotten the following serrasalmus species off live feeders so this is not an isolated case: geryi, rhom, manueli, elongatus, brandti, purple spilo, gold spilo, and scapularis. It seems kinda harsh but if you don't give a healthy piranha a choice of live foods, then I strongly believe that they will eat the prepared foods.
