Discus are fish that are best kept in small groups of six to eight individuals. The genders become apparent from the fishes' behaviour when they pair off. Males are more aggressive and protective than females. Visual differentiating them is more difficult as male and female discus fish look very similar. As juveniles they are practically indistinguishable. Mature discus have a few physical differences and it is possible to visually differentiate them if you look closely.
Compare the sizes of the fish. Males tend to be slightly large than females.
Examine the dorsal fin. The females have rounded dorsal fins while the males' dorsal fins are more pointed.
Look at the mouth. Male discus have thicker lips than females.
Look at the anal tube, if the fish are spawning. This is located between the fish's anus and its anal fin. In females the anal tube is broader, blunter and larger than in males, whose anal tube is relatively small, narrow and pointed.