Male angels are more aggressive and territorial than females. When they become juveniles, they will engage in pushing each other around by their mouths to impress the females.
Angels need a minimum of five gallons per fish. I prefer ten gallons. Anything less than five is overcrowding. Angels lay hundreds of eggs so both males and females will become very territorial when they are overcrowded. They want the other ones to go away so they can get down to business!
They will be most aggressive when they have:
Decided to lay eggs
They are laying eggs
They are protecting their eggs
Their eggs are hatching
Their babies are swimming
Watch this video of Lindsay Buckingham in a war lock with Pipsqueak Littlefish. Pipsqueak was told by the other fish that Lindsay and Stevie had opened a sushi bar. When he went to check it out, Lindsay warned him away - and Pipsqueak didn't listen.
Angels will waggle their vertical fins when fussing at each other. Watch the body language even before they lock lips!
Notice the tetras, the corydoras and even the pleco swim right into the middle of the fight and the angels take no notice of them.
By the way, no angels were hurt in the filming of this documentary. Pipsqueak and Lindsay made up a few minutes later.