as already said....just be sure to scoop the uneaten part sof the dwad goldfish that are on the bottom of your tank...it'll ruin your water conditions
for future reference...u may want to set up a quarintine tank for the feeders that you are going to feed to your piranha or any other predatory fish that u may have.
the quarintine tank doesnt need to be anything fancy...just use a 10g tank (which u can buy for less than $10), stick a filter on there & personally I would put an air wand in the tank since the tank will more than likely be very overstocked with goldfish. goldfish are messy and seem to mess up water conditions very quickly...especially when dealing with a tank with that many goldfish in it. just do more frequent small water changes and everything should be just fine.
the 'quarintine tank' is used to make the feeders that you are feeding to your piranha healthy and disease free. buying them from your lfs, bringing them home & just tossing them in your tank is risky because u dont know what diseases/parasites you could be introducing to your tank & fish.
just something u might want to think about