In the case of adult angelfish, trained observers can identify the sexes during the spawning season. Outside of the spawning season, sex determination is possible but uncertain. The sex differences are only weakly developed, but typical for perch.
A sure identification feature is the genital papilla. In females it is about 2 – 3 millimeters thick, rounded, blunt or flattened and about 2 – 4 millimeters long.
In males, the genital papilla is oblong, about 1 millimeter thick, 1 to 2 millimeters long and tapering to a point. In the male, only a small, pointed something sticks out; in the female, the papilla can be 6x as thick.
In both sexes, however, the genital papilla can only be seen a few days before the eggs are laid or only very briefly during the spawning process.
The sexes are best recognized during egg laying. The eggs placed by the female are fertilized by the male rubbing his abdomen over them. However, eggs are also laid if there is only a single female or two females without males in the aquarium. So if the other angelfish doesn’t brush over the eggs or even lay eggs itself, it won’t be a male.
Males also have a slightly more beefy head than females. Older males get a hump on the forehead. which is curved outwards like a melon. Females tend to have a straight forehead. However, these differences are hardly noticeable. Males are slightly larger than females.
Females are usually rounder in the abdomen. Females often have a spawning approach visible from the front. Some females make cracking noises when courting the male.
A red iris is not a sex characteristic in angelfish, although this is sometimes claimed.
Because it is not easy to distinguish between the sexes and angelfish are demanding when it comes to choosing a partner, a small swarm of 4 – 5 young should be bought if the aquarium is large enough.
Only at spawning time do scalars form territories.