piranhas dont need huge tanks for the general population of them.....depending on the size many of the serrasalmus species can do fine in a 20L or a 40 breeder....rhombeus take years to grow to monster size so if someone was to purchase one at 3-4" it could take awhile for it to outgrow a 20L.....now if you were keeping a shoal of pygos than you could get into some large tanks.....3 red bellies could live happy in a 75g for life.....its usually 20g per fish rule when it comes to pygos......thats just my take....so for all u future piranha owners dont like this topic scare you away.....depending on the species of piranha one could live in a smaller size tank for a good amount of time maybe not life but a good period of time.......