I once had 2 small piranha when I was younger, about 2 1/2 inc. Whit I did to feed them was just to put a littel piece of raw meat, and tied it to some fishing line and hung it into tank. When they were done eating I threw it out. more humane then fedding gold fish to them depending on the size of the piranhaa 2 small piranh will not eat a whole gold fish.
yeah piranha's do, do better in 3 or more weather its a red belly, black, or white I had 3 or each all in 1 tank and it worked out great! my females (4) where approx. foot n half long and my males (5)where about 1 foot but they where alos in a 3000 gal custom built tank so there was plenty of room oh yeah and they all addopted this 1 feeder fish as "one of them" costed me more to feed this dam feeder that was almost 2 feet long then it did my piranahas and they all got along awsomely! lol although my "giant feeder" tend to beat on my piranaha's which was hilarious! so yeah be prepared to upgrade you tank a lot if you plan on keeping your piranha(s) as they are like Oscars and out grow tanks fairly fast!