Piranhas In My Biology Class

[AMS]

holy crap. the tanks too small to house two piranahs. pirranahs will be kept in a tank for a good year liveing together peacefully then one day the head and gills of one of them can be found floting at the top of the tank. its a really bad idea to keep piranahs together in a tank that size.....

I dont know where you read that, but a shoaling species such as red bellies won't all of a sudden rip each other to shreads. It is suggested to keep 20 gallons per red belly, 55 gallons you could house two with enough territory, but they will lack swimming space.