If the new one is alot smaller than the established one, then "she" (bigger one) is showing him the smaller one whom is boss. I would try and keep the lights off for a couple more hours. She may end up killing him anyway. It's always a good idea to pick out 2 from the same tank at the LFS, again 1 big, 1 small. I picked out two small ones and there is clearly a dominant one and she has gotten alot bigger than the other. Hope that helps.