Is it always nessasary to add two that are of different sizes, as in one bigger then the other. I added two black ocellaris to my nano at work as juveniles, assuming one would out grow the other and become the dominate female. It seems after almost 2 years together they are almos tthe same size and one dominates the other into the corner. Yet the fins are not injured at all the pecking seems a bit rough for mating or deciding who the female is deal. Good thing the tank has a lid on it or one might throw the other out of the tank. I am thinking of taking one home and then adding a new young black ocellaris to each tank.