First off there's no s. niger only s. rhombus. They are white at birth and they don't darken up until they are around 9 inches or so. So thats why theres a white piranha and a black piranha. They just didn't know it was the same fish going through a change. But I won't own a black rhom until it was at least 10 inches or so. Rhoms are usually shy under 10 inches. So having them big can help out making them aggressive or just look at my thread in tutorials how to make a rhom aggressive.