Guides - Identifying Sex.

Not knowing what species of fish you have makes it difficult to answer this question. Depending on the species, determining the sex ranges from easy to nearly impossible. Here are some tips for sexing some common species of aquarium fish. This is by no means an exhaustive list, but will give you a general overview of the differences between the sexes

Angelfish
Angelfish are nearly impossible to sex accurately. Occasionally fully mature males will show a bit of a nuchal hump, which is a bump on the head just above the eyes. However, don't count on it. If you are seeking to establish a mating pair, the best way is to purchase a half dozen immature angels and raise them together. When they are mature they will pair off, and you will get at least one breeding pair out of the group.

Betta
Bettas are a species of fish that is easy to sex. Males have the long flowing fins and brilliant colors that owners find attractive, and are what is usually sold in shops. Females are drabber and have short rather stubby fins. It is not always easy to find females for sale in pet shops. If you are seeking a female, ask the shop owner or manager if they can order one for you.

Catfish
Generally speaking, catfish sexes cannot be distinguished. Likewise, many species of catfish have been difficult to breed in captivity. The notable exception are the Corydoras species, which can be bred in aquariums.

Cichlids
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. 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, sometimes 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
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.

Gourami
Gouramis are also among the species of fish that are not as easily sexed. Males and females often are similarly colored and shaped. However, there is one fairly universal sexual difference seen in most Gourami species. The dorsal fin is long and 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 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
Perhaps the easiest category of fish to sex 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
Tetras are not easily sexed, 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. Because Tetras are schooling fish, breeding pairs can be obtained simply by purchasing a small school of them at one time.