count your blessings one by one! most people here have to spend exhorbitant amounts of time and effort to get predators to stop only eating feeder fish. really, if you've already got your piranhas eating whitebai/krill/pellets/etc -- don't get them stuck on feeders!
unless you're breeding them for yourself, feeder fish are typically diseased, malnourished, over-priced and generally just not worth it. feeder goldfish are all those things plus fattening, so they should not be offered regularly even if you are breeding them yourself.
i say leave well enough alone. especially since i think i remember your pirhanas being a bit space deprived--giving them a taste for fish is likely to increase their aggressive tendancies.