[Fishfirst]

Piranha aren't the hardest fish to keep. They tend to be pretty darn tough. But you need to realize, piranha aren't by any means the run of the mill platies, swordtails etc. They are SHY! Especially when they are between the size of 2 inches and 6 inches. In a school, they are bolder, but not increadibly. 3 in a 55 gallon is a good idea, 3 spreads the aggression, makes them feel a little bit more comfortable, but there are also problems... a 55 is by far the minimum size tank. These babies get 12 inches in length, thats as wide as a 55 gallon! Once they get in the 10 inch area, I'm going to give two away... its just gonna be so over crowded. I've found that they also like a lot of cover, and not very bright light... anything that shades out the light... (such as those fake lillies from wally world) make them more comfortable too.
Feeding piranha can cost you a lot... if you do it the wrong way. Goldfish are like piranha cheeseburgers. Not healthy, and expensive. Feed them a feeder thats been quarenined for a good period of time is alright once in a while... but their main diet should be chunks of raw shrimp, raw fw fish, and a good flake diet.