Goldfish aren't the best thing to feed your piranha.
"Goldfish, minnows and other members of the Cyprinid family (Carp-like fish) should be avoided, as these fish contain growth-inhibiting hormones (Thiaminase/Vitamine B1 inhibitors) that could negatively affect the fish's health and development."
COURTESY OF www.piranha-info.com
You're better off to go with convicts or tetras, or other like fishes.
Plus, the conditions that goldfish are kept in the stores are encouraging disease, and that's the biggest reason "feeder" goldfish don't live past a few days.
However, if goldfish are what you prefer, be sure to feed a balanced diet, including veggis, since goldies are omnivorous (and appreciate a snack of peas about once or twice a month) and quarentine, the ones that don't look so good after a week, cull them (flush them, don't feed)
Feeding live is complicated, but, as I stated before, it does have it's benefits, and if I can ever get my convicts breeding, whatever my fiance doesn't keep, our piranhas get.