Poecilia wingei - Gonopodium

Choy Heng Wah


Depending on the species, determining the sex of a fish ranges from easy to nearly impossible. Knowing the sexual differences in aquarium fish is important when trying to breed fish, and also for choosing the proper balance of fish for a community aquarium. Although not all fish can be easily identified, these tips will help you recognize the sex of many of the common species of aquarium fish.


Determining Fish Sex

Here is how you can figure out the sex of common aquarium fish.This is by no means an exhaustive list but does provide a general overview of the differences between the fish sexes.


Angelfish are extremely difficult to know accurately which is which sex, particularly when they are young. Occasionally fully mature males will show a modest nuchal hump, which is a bump on the head just above the eyes. Don’t count on it being there in every case, though. The best way to establish a mating pair is to purchase a half dozen immature angelfish and raise them together. When they are mature enough, they will pair off, and you will have at least one breeding pair out of the group. Once they begin mating, it will be quite obvious which one is male and which is female, as the female will be the one with the ovipositor laying the eggs.


Bettas are a species of fish that is quite easy to identify. Males have the long flowing fins and brilliant colors that owners find attractive. Male Bettas are the ones usually sold in shops. Females are not as vividly colored and have short, stubbier fins.

It is not always easy to find female Bettas for sale in pet shops; if you can’t locate one, ask the shop owner or manager if they can order one for you.


Generally speaking, catfish sexes cannot be distinguished. Many species of catfish have not been breed in captivity. The notable exception is the Corydoras species, which has often been breed in captivity.


Cichlids are such a diverse group that it would take a small novel to give specifics for knowing the difference within each species. While many are not easily differentiated, there are a few rules of thumb that apply to quite a few Cichlid species.

Males are often slimmer, but larger than females, and are more vibrantly colored. The dorsal and anal fins of the male are more pointed, larger, and more flowing than the female. In many species, the male will display egg-shaped markings on the anal fin known as egg spots. Some males have a bump on the head, referred to as a nuchal hump. Although females can also develop a nuchal hump when spawning, it is never as prominent as that of the male. Typically. the dominant male will have a larger nuchal hump than other males.

Although the above general rules apply to many species of cichlids, if you are considering breeding them, do your homework on the specific species before seeking a breeding pair.




Barbs and other members of the cyprinid family are rather difficult to tell apart. Differences will vary by species, but generally, males are more intensely colored and slimmer than females. Since most cyprinids are schooling fish, one way to obtain a breeding pair is to purchase a group of them.


Gouramis are another species of fish that are not easily identified. Males and females often are similarly colored and shaped. There is, however, one fairly universal sexual difference seen in most Gourami species. The dorsal fin is long and comes to a distinct point in males, while females have a shorter, rounded dorsal fin.

In addition, certain species of Gourami show color variations between the sexes. The male Pearl Gourami has a deep red-orange coloration on the throat and breast. The male Moonlight Gourami has orange to red coloration of the pelvic fins.

Livebearing Fish


Among the easiest of all fish to tell apart are the livebearing fish. Males are usually smaller and more colorful than the females. They also possess an external sexual organ, the gonopodium, which makes it easy to differentiate males from the females.

The gonopodium is a modified anal fin which is used to fertilize eggs. In the male, the anal fin is rod-shaped, while the female has a traditional fan-shaped anal fin.



Tetras do have some differences between sexes, which vary based on the species. The females are a bit larger and plumper than males. Males are often more vibrantly colored and may have longer fins than their female counterparts. Tetras are schooling fish, so breeding pairs can be obtained simply by purchasing a small school of them at one time.

Original article from thespruce.com