I have 3 red belly piranhas, i bought them when they were babies about the size of a nickel and now they are medium size. my tank is a 15 gallon tank (i know, very small) but i have had no problems with it. my piranhas are fine and happy. they swim around all the time and eat normally. They have never taken bites out of eachother but that is because i keep them well fed. if you were to put piranhas in that tank, make sure you put in more than 2 prefirably an even number and put lots of hiding spots. i also have a pleco in my tank because piranhas are very dirty.. you shouldnt put other fish in with the piranhas.. they WILL get eaten. Although plecos are armored and i think its worth taking the risk. my piranhas dont even bother trying to eat him.