The general concensus is three in a 55 gallon tank. But keep in mind that it is approximately 13" front-to-back (less on the inside, most measurements are the outside dimensions) and a full grown piranha will not have much turning room. Granted it takes a long time to have a 12" nattereri, but still something to consider. Have you considered a 50 or a 40 gallon breeder? They offer more space though a little less height, which isn't really an issue in home aquariums.