I have 5 red belly piranhas, and one black piranha separated in the following way: 1 4" in 20 gallon, 2 6-7" in 38, 2 3" in 20, and the one black in a 29. The one 4" fish in the 20 gallon is mine and my wifes favorite, and he is called simply "fishy". He is an absolute glutton and will eat just about anything, and in any quantity, so I have to kinda regulate how many feeder fish he gets at one time. I had just arrived home from the fish store with an additional saltwater fish for one of our other tanks, and about 30 feeder goldfish in a bag. I wanted to spend a few minutes acclimating the saltwater fish, so I put the feeder fish bag into "fishy's" tank, which was the closest so they could acclimate while I worked on the saltwater. 5 minutes later, after being satisfied with the saltwater fish situation, I returned to the feeders. To my shock I noticed about 15 fish in the water in fishy's tank and another 5 or so in the bag, and one very fat piranha. Upon closer inspection I found that fishy had bitten one very clean hole in the bag just big enough for the feeders to swim out of, only to their demise. Needless to say, the other piranhas went a little hungry and I learned to keep an even closer eye on fishy.