The problem with adding new angels to a tank that has been housing one angel is territory. There is a chance that the existing angel has established territories in that tank, although it may be a little young for that. What would help is taking out the angel you have, and rehoming temporarily in another tank, or even a bucket while you rearrange the tank & add the new fish. Angels key their territory off of objects in the tank, rearranging while the fish is not in there will destroy old territories.
If I could figure out why some angels, even those out of the same spawn, are peaceful & others are aggressive I would write a book. Temperment is one of the traits that people breeding should be more concerned with, from my experiences & those of others black angels do tend to be more aggressive than others. Keep a divider handy just in case you do have some aggression issues.
