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How Do I Tell the Sex of My Fish?

You don’t have to have intentions of breeding fish to be curious about the sex of your fish. Knowing the sex of your fish can help you understand the behaviors your fish exhibit. Depending on the species, determining the sex of a fish ranges from easy to nearly impossible. These tips will help you sex common species of aquarium fish. 

This is by no means an exhaustive list although does provide a general overview of the differences between the fish genders.

Angelfish Gender

Angelfish are nearly impossible to sex accurately. Occasionally fully mature males will show a modest nuchal hump, which is a bump on the head just above the eyes. However, don’t count on it.

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.

Betta Gender

Bettas are a species of fish that is easy to sex. Males have long flowing fins and brilliant colors that owners find attractive. Male Bettas are what are typically 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 a female Betta, ask the shop owner or manager if they can order one for you.

Keep in mind that juvenile fish may not display sexual differences. Ideally, compare mature specimens of the same species and color and compare multiple factors rather than using a single trait to determine sex.

Catfish Gender

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

Females are usually more robust when viewed from above and are usually bigger.

Cichlid Gender

Some Cichlids are easier to differentiate because males are typically more colorful than females. Like the Maylandia estherae below:

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

 The only sure way to determine the sex of egg laying fish is to do what the experts call “venting” the fish.

You must remove the fish from the water (net it out of the tank) and turn it upside down (hold it very gently, they can be hurt easily) so you can examine the vent holes in the anal area. Only do this over a bucket of tank water so if you drop the fish or it flips out of your hands, it will fall into the water in the bucket.

You will see that the females will have a rounded vent (hole) and the males will have a tear-drop shaped vent. If the fish are young, it will be very difficult to discern the difference in shapes.

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. Some males have a bump on the head, referred to as a nuchal hump.

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

Cyprinids / Barb Gender

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

One way to know is that female barbs are quite larger than male ones. Now, male tiger barbs will show bright red noses when mature. The dorsal fin will have a red line above the mainly black fin and his ventral fins will turn bright red. 

When in spawning condition or fighting for a higher “rank” in the shoal, the males’ stripes turn a slight metallic green color. The tips of his upper body scales turn almost black and shine bright orange in certain light.

The females, however, keep their pale yellow noses (may turn slightly pale red at times, which makes it tricky to determine their sex in the store). They only show a small area of red at the tip of the dorsal fins, and the ventral fins will stay pale red. In females, their stripes stay black.

Gourami Gender

Gourami’s are another species of fish that are not easily sexed. Males and females often are similarly colored and shaped. However, there is one 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 to the dorsal fin differences, 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 Gender

Perhaps the easiest fish to sex are the Livebearing fish. Males are usually smaller and more colorful than females. They also possess an external sexual organ, the gonopodium, which makes it easy to differentiate males from females.

The gonopodium is a modified anal fin that is used to fertilize eggs. In the male, the anal fin is rod-shaped, while the female has a traditional fan-shaped anal fin (see photo above).

Tetra Gender

Tetras are not easily distinguished but do have some differences, which vary based on the species. Generally, 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 and breeding pairs can be obtained simply by purchasing a small school of them at one time.

Any fish that gives birth to live young is usually easier to sex. These are known as sexually dimorphic. These include the family, Poeciliidae, which are mollies, guppies, swordtails, platies, and wags. They are easily distinguishable by the gonopodium, which is male modified anal fin used for copulation.

Telling Koi Carp apart for example is almost impossible until the female is ready to spawn.


Ultimately the best way to tell the gender of fish is to obtain some information on the species you are interested in. If you study their behavioral differences, as well as physiological, make up you may be able to make an informed decision.

All fish species have their own methods of telling males from females, but if you’re not a fish, you might not recognize them! If you are interested in breeding your fish, we recommend contacting a professional breeder for some tips.

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