One of the biggest question asked of the discus breeder is "how do I determine the sex of my fish?" There are very few easy identifiable identifiers in this process. Here, we will discuss the methods used by some of the top breeders.
In juvenile fish, determining sex is almost impossible. It is only when they begin to pair off that an opportunity arises to help in the determination of sex. Juvenile fish, both male and female, have a rounded dorsal fin, and it is not until they begin to mature that a difference can be detected. As it is never wise to excessively handle the fish, close observation is in order to aid the breeder.
In Allnut Enterprises' King Discus Hatchery, for example, it is an easy process to determine who is who, as we have observed these fish for a while, and can determine the sex of the pairs we own. This would be true in any hatchery. But to the uninitiated or casual observer, this would not be easy to do.
Some of the identifiers: The male will have thicker lips to aid him in his fight to protect the female, and will be more aggressive. He will be larger than the female, his forehead is thicker, and we have observed that if the discus are a bit shy, the male will have a tendency to stay between the female and the observer.
The dorsal fin of the male will be pointed, and the female's dorsal fin will be rounded. Note that in juvenile discus, this is not apparent.
The breeding tube of the female, between the anus and anal fin, is broader and rounder than the male, and will have a blunt tip. The male, in turn, has a smaller, sharper breeding tube. Be aware that this is only evident during spawning, and should be closely observed.
It has been said that the male discus fish will tend to have a less intense color and more pattern while the female tends to be more colorful but with lesser pattern. I disagree, because too many variables are in place here, such as the health of the discus, the water parameters, and feeding pattern.
In an interesting article by Jeff Richard, [http://www.aquaworldnet.com/dbws/sexingdiscus.shtml] he discusses an article from Diskus Brief, a German publication, which reports a very successful way of determine sex of a discus by using simple geometry. Jeff reports, and I quote: "Picture a discus facing to your left … you would be looking at its side. Find the Dorsal (Top) and Anal (bottom) fins and look where the fins slope down toward the Caudal (tail ) fin … make sure you're looking at the fins after they have curved back toward the tail. The Dorsal and Anal Fins become (almost) straight after the fins curve down (or up) toward the Caudal fin … extend an imaginary line along this straight section of the 2 fins back toward the tail which just touches the Dorsal & Anal Fins past the Caudal Fin. These two imaginary lines should intersect behind the fish. The key to sexing the fish is where the lines cross the Caudal fin. If they pass through the Caudal fin, the fish is most likely a FEMALE. If they miss or just touch the Caudal fin, then most likely it is a MALE. " Thanks, Jeff!
Sexing Discus is difficult at best. The easiest way to do so is to raise a group of at least six to eight discus, and allow them to pair off when ready. It is a beautiful sight to see this happen, and makes the hobby well worthwhile.