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 by sex, 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 it 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 should 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 she will be the one with the ovipositor laying the eggs. The ovipositor is a short tube from the female's vent that she uses to lay her eggs. It has a rounded tip. The male will use a pointed tube that extends from his vent to fertilize the eggs after they are laid by the female. Both parents will take turns hovering over the eggs, fanning them with their fins to oxygenate them and keep them clean.
Bettas are a species of fish that is quite easy to identify their sexes. 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, but some male betta varieties can also have short 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. Male bettas must be kept separate from each other and from the female until she is ready to breed. The male builds a nest out of air bubbles on the surface of the water and then entices the female to lay her eggs under it, wrapping his body around hers to fertilize the eggs as she lays them. He then collects the eggs into his mouth and places them into his bubble nest. After breeding, the male chases the female away and takes care of the eggs and the babies. The female must be removed or she will be attacked by the male.
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. In the Corydoras catfish, the female is usually a bigger-bodied fish than the males. The males will chase a gravid female (full of eggs) around the tank and then she will lay the eggs on a flat surface, such as a plant leaf, smooth rock, or even the aquarium glass. The males swim over the eggs and fertilize them, then the eggs are abandoned.
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-bodied than females and are more vibrantly colored. The dorsal and anal fins of the male are more pointed, larger and more flowing than in 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. In some species of cyprinids, including goldfish, the males will get small white bumps (nuptial tubercles) on their head, operculum and possibly the spine of the pectoral fins during the mating season. The males will chase the female who will lay her eggs and the males fertilize them as she deposits them. After breeding, usually there is no care of the eggs, and the parents may even eat them if they are found.
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. Like the betta, many gouramis will build a bubble nest, but both the male and female are involved in taking care of the eggs.
Among the easiest of all fish to tell apart are the livebearing fish, such as the guppies, platys, mollies, and swordtails. 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 the females: In the male, the anal fin is rod-shaped, while the female has a traditional fan-shaped anal fin. The gonopodium is used to fertilize eggs inside of the female fish as the male swims alongside the female. The internally fertilized eggs will hatch inside the female and then she will "give birth" to the babies.
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. Generally, they breed by the males chasing the females and fertilizing the eggs as she lays them in the plants or scattered among the rocks. There is no parental care to the eggs, which may be eaten if discovered by the fish.
Freshwater Angelfish. Rosamond Gifford Zoo
Betta Fish | Advocating For Proper Care & Information. Bettafish.Org
The Aquarium Environment: Selecting Compatible Cichlids. Massachusetts Institute of Technology