With angelfish, unless you have a mated pair for breeding, they are better in a small group. Fish are living creatures, just as dogs and horses...and common sense tells us we can't keep a horse in a bedroom or the back yard (unless you're on a farm of course
), nor a dog in a small wire cage in the back yard. And my point here is that an animal must be fully understood and its needs provided for it to be truly healthy and happy. One might argue as to how an animal experiences "happiness" but there can be no argument on the healthy side, and the two are connected in humans and domesticated animals so wild animals should be little different.
Angelfish are shoaling fish; in their habitat they live together in groups, and they establish a hierarchy within that group. This is how they are programmed by nature. To deny any fish what it naturally expects is not going to be in the fish's best interest, and its health is most likely affected somehow. This is why so many of us keep recommending research to know the needs of each species before acquiring it. When I was young (and that was several decades ago
) we had one of those round bowls on the buffet in the dining room, in which we had 2 goldfish. The goldfish never lived long, maybe several months, and were continually being replaced. Goldfish can live for decades too, and should--if they are healthy and being given what they need in terms of space and care. When we decide to acquire any fish, we have a responsibility to understand its needs and provide for them.
It is not easy to discern the sex of angelfish until they are mature enough that the breeding tube is visible; the female's is thicker. But angels do not always mate with any other fish. An incompatible pair can result in the male killing the female. Another reason for a small group, so they can select their mates.
On the African Butterfly male/female, the male has a distinct notch in the anal fin, whereas the female's is rounded. Seen together, this is unmistakable.