Can I eat it?
Can it eat me?
Can I have sex with it?
It seems that angelfish have three purposes in life in the wild: to reduce the population that's smaller than they are, to be food for fish that are bigger than they are, and to breed.
In the aquarium, an ideal situation neither gives them smaller fish to eat nor bigger fish to eat them.
By the time they are teenagers (within six months) they are looking for a mate.
The female shows interest by cleaning a vertical surface in front of the male. She's saying, "I'm ready to lay eggs here."
The male shows interest by showing how fierce a protector he can be.
Because when those eggs are laid, he is going to have to protect them -- even if there are three hundred or more. When they hatch, he must continue to protect them. And he must protect them when they are swimming but too small to defend themselves.
Here is Lindsay Buckingfish protecting the eggs Stevie Fishnick laid on the filter. Pipsqueak Littlefish, the band manager, sees the eggs as a sushi bar. But Lindsay tries to ward him off by a display of fins. When that fails, he goes in for Pipsqueak's lips. They lock. Lindsay wins. And a short time later, they're friends again - as long as Pipsqueak stays away from the eggs!