Yesterday I spoke about dither fish, other fish that you might want to add to the tank to encourage shy angels to come out and play.
Some folks use target fish to help keep aggression down between angelfish.
If you have too many male angelfish in a tank or you have overcrowding, the angels can become more territorial and aggressive. In the case of overcrowding, the only recourse is to get a larger tank or move some of the fish to a separate tank.
But something interesting happens when fish who were aggressive with each other feel a threat from outside: they bound together. You can see this happen when cleaning a tank. My tanks are so large that I clean one side one week and the other side the following week, removing about 20% of the water each week. When I am working on one side, the angels all gather on the opposite side. Territories are forgotten.
The same thing happens when introducing another type of fish to the tank. Sometimes these target fish give the angels something else to think about and they stop picking on each other.
Some considerations if you try this:
The target fish must be faster than the angelfish. You really don't want casualties. Otherwise, just buy them some shrimp or feeder fish.
The target fish can not be aggressive toward the angels. Barbs, for example, love to nip at an angel's fins and being much faster than the angelfish, they can make your angels truly miserable.
I've found that my angels largely ignore other fish in the tank. They are curious when seeing something foreign for the first time - such as a pleco or a snail. But once they've decided they can't eat it, it can't eat them and they can't have sex with it, they tend to leave it alone.
If you have target fish in your tank, what type are they?