Ive owned reds and from my and others I've read about experiances 15 gallons per adult red in minimum but aggression might plauge you, 25 g per red should allow them enough space not to be as tempramental twords each other, and let you enjoy them with less worry.Also they REALLY like to feel they have cover, I highly suggest a lot of plants plants real or fake. Pirahnas are nervous fish and what scares them they ushally try to kill and eat. The more comfratable the better theyll live together. Good luck.