How to Tell If a Discus Fish Is a Male or a Female?
1 Look at the size of the fish. Generally, the male discus fish will be slightly larger than the female if both fish are mature.
2 Study the lips of the fish. Because males use their lips to fight, a male's lips are usually larger than a female's lips.
3 Compare the dorsal, or top, fins. Females have a dorsal fin that is round, while males have a pointed dorsal fin. Prior to reaching sexual maturity, all the fish have rounded fins.
4 Study the behavior of the fish. A male discus is usually more aggressive than a female discus. You may notice him nudging other fish away from the female with whom he is trying to breed. In some cases, he may seem to protect her by placing himself between her and other fish in the aquarium while flaring his fins to make him look larger.
5 Observe the breeding tube during spawning. It is located between the anal fin and the anus. The female's breeding tube is rounder and wider than the male's breeding tube, since she has to lay the eggs.
6 Note the color and patterns of the discus fish. If all of the fish in your aquarium are healthy, the male fish will have more patterns and less color than the female. However, this method shouldn't be relied upon by itself, as the water quality and subtle changes in health can affect the coloring of the discus fish.