If you are going to feed them just goldfish, you should atleast have a seperate "feeder tank" if you don't already. A tank that you can put the feeders in when you first buy them, this way you will know if they carry any dieases before you put them in the tank. Also with the good food you will be giving them, it will really up^ there nutrional value for the piranhas. I do this for my barracuda.