Yes, interesting, I believe the idea of only adding in pairs makes a lot of sense based on my memory of their behaviours. Although a single angel can get along by itself (a one, or a leftout of a three, whatever), I think it would seem happier and less stressed if it had a chance to glide about the aquarium as a pair. Its almost as if, for angels, even if they are not behaving as a breeding pair, they relax in a pairing situation kind of like shoaling fish relax at some minimum shoal size, wouldn't you say?
~~waterdrop~~