They look like colorful, finned plates languidly swimming around in your aquarium, but they're discus fish. The question is, who is who? You can't name your fish if you don't know their gender. You can figure out whether your discus fish are males or females a few different ways, but you'll have to pay close attention.
Waiting It Out
It's tough to figure out the gender of juvenile discus fish. As adults, these beautiful swimmers will reach around 8 inches in diameter and display some eye-catching color combinations. However, as juveniles, they all tend to look alike because they haven't developed their adult colors or reached their full size. At about 6 months old, discus fish are about half-grown and may start to show hints of coloring. Discus males' colors, theoretically, are duller and more patterned than females', but aquarists including AquariumLife.net creator Keith Pardee believes color is not a particularly accurate method of sexing because of numerous variables, such as diet, water quality and health, that can affect color. You can't count on the dorsal fin to be a reliable indicator, either, as all juveniles have rounded dorsal fins regardless of gender.
Time will tell the tale of which of your discus fish are males and which are females. Like many other animals, male discus fish are larger than their female counterparts. Males have thicker foreheads and lips. As your discus grow, you'll be able to separate the men from the ladies by the shape of their dorsal fins. A male's dorsal fin becomes pointed, while a female's maintains its round shape and is usually shorter than a male's. You can tell the difference at spawning time, too; the female of the species has a larger breeding tube, located between her anus and her anal fin. Hers is broader and rounder than the male's, which is sharper and smaller.
Gender Behavior Patterns
If you spend enough time watching your fish, you may pick up on some behavioral cues to inform you of gender. As discus fish mature, they tend to pair up, male to female, to mate. The males tend to be aggressive, particularly when it comes to protecting their mates. If you're gazing into your aquarium and find one of your fish has blocked your view of his partner, it's probably a male, who will often come between his shy female mate and any outside observers.
Math to the Rescue
If you didn't think your high school geometry class was going to be useful later in life, guess again. An article in "The International Discus Journal" revealed a method of using intersecting tangent lines. Visualize your discus facing to your left as you look at his side. Locate the dorsal and anal fins and imagine you're drawing a straight line along the edge of each fin, extending back toward the tail fin. The lines will intersect behind the fish. If the lines pass through the tail fin, the fish is probably a female; if they miss or barely touch the tail fin, it's likely a male.