thats not true. there are two main groups of piranhas. serr. and pygo. (not full name cause i dont know how to spell lol) serras are general kept alone unless kept by advanced hobyest. its hard to keep them together because they are so teritorial and agressive. some are easyier then others to keep together. then pygos are group fish. a single pygo will general but not always be skittish and shy. you cant keep just 2 together because one will pick on the other to much and end up killing him. (once again this is in most cases) you need a group of 3 or more. the bigger the group the more agressive the shoal will be. red bellys are the most comin. then there are terns witch are classified as red bellys but are much more rare and cost alot more. caribes and piraya are more agressive and rare and much more exspencive then red bellys. red bellys can be bred in captivity witch makes them very cheapp but no one yet has bred the other species. piranhas are fresh water fish that like then temp around 78-82. the warmer the water the more agressive. like if you were trying to keep serras together you might keep the temp at 74-76 so they wont be really active and agressive twards eachother. but red bellys you might keep its at 84-86 too make them really active and agressive. when feeding piranhas it best to feed frozen or fresh dead food. smelt, shrimp, prawns, beef heart, fish fellet, and many other white meat fish are good for piranhas. red meat fish is to faty but can be fed to them as a treat. mamals meat is to faty to. if you start out with baby piranhas like .5 to 1 inch red bellys it is best to feed them three times a day until they get larger then you can slow down to every other day or less often. baby piranhas will eat flakes pellets froozen blood worms or brine shrimp and they cann also nibble on the other stuff i listed. im sure i forgot something or messed something up so if you have anyquestions just ask. cya