Depending on the species of fish, determining the sex of fish can range from easy to nearly impossible. Knowing the differences in sex among ornamental fish is important for fish keeping and also to create the right balance in the aquarium. Although not all fish species have easily distinguishable sexes, these tips will help you identify the sex of many common ornamental fish species.
Determining Fish Sex
Here’s how you can find out the sex of common ornamental fish species. This is not a comprehensive list but provides an overview of the differences between fish sexes.
Angelfish
Angelfish are extremely difficult to determine the exact sex, especially when they are small. Sometimes, fully grown males will develop a small bump on their head, which is a bump above the eyes. The best way to distinguish them is to buy some juvenile angelfish and raise them together.
When they mature enough, they will pair up. When they start breeding, it will be quite clear which is male and which is female as the female will be the one laying eggs. The ovipositor is a short tube from the female’s vent used for laying eggs. It has a rounded end. The male will use a long pointed tube extending from his vent to fertilize the eggs after the female lays them. Both parents will take turns swimming around the eggs.
Betta Fish
Betta fish are a species where it is quite easy to determine their sex. Males have long fins and vibrant colors that attract owners. Male Betta fish are often sold in stores. Females lack the bright colors and have shorter, stockier fins, although some male Betta strains may also have short fins.
Male Betta fish must be kept separately from each other and with females until they are ready to breed. The male builds a bubble nest on the water’s surface, then entices the female to lay eggs underneath it, wrapping around the female to fertilize the eggs as she lays them.
Afterward, the male collects the eggs in his mouth and places them in his bubble nest. After breeding, the male chases the female away and cares for the eggs and fry. The female must be removed or she will be attacked by the male.
Catfish
In general, the sex of catfish cannot be distinguished. Many catfish do not breed in captivity. Notably, the Corydoras species often breed in captivity. In Corydoras catfish, females are usually larger than males.
Males will chase a female full of eggs around the tank and then she will lay eggs on flat surfaces, such as leaves, smooth stones, or even the glass of the aquarium. The male swims over the eggs and fertilizes them, after which the eggs are abandoned.
Cichlids
Cichlids are a very diverse group of fish with many different species. While many species are not easy to distinguish, there are some general rules that apply to certain Cichlid species.
Males are usually slimmer but larger than females and have more vibrant colors. The dorsal and anal fins of males are sharper and larger than those of females. In many species, males will display egg spots on their anal fins known as egg spots. Some males have a bump on their head. Although females can also develop a bump when breeding, it is never as prominent as that of males. Typically, dominant males will have larger fleshy bumps than other males.
While the above general rules apply to many Cichlid species, if you are considering breeding them, do your homework on the specific species before seeking out a breeding pair.
Cyprinids
Different species of the Cyprinid family are quite difficult to distinguish by sex. The differences will vary by species, but generally, males are darker and slimmer than females. Since most Cyprinids are schooling fish, one way to obtain a breeding pair is to buy a group of them. In some Cyprinid species, including goldfish, males will have small white pimples (breeding tubercles) on their heads, bellies, and possibly even on the spines of their pectoral fins during breeding season.
Males will chase the female to lay her eggs and the male fertilizes them as she spawns. After breeding, the eggs are usually not cared for, and the parents may even eat them if they are found.
Gourami
Gouramis are another species of fish that are not easy to identify. Males and females often have similar colors and shapes. However, there is a fairly common sexual dimorphism seen in most Gourami species. Males have longer dorsal fins with distinctive points, while females have shorter, rounder dorsal fins.
Additionally, some Gourami species show color differences between the sexes. Male pearl gouramis have a deep orange-red color on their throats and pectoral fins. Male three-spot gouramis have orange to red ventral fins. Like Betta fish, many Gouramis will build bubble nests, but both males and females participate in caring for the eggs.
Livebearing Fish
Among the easiest fish to distinguish are livebearers, such as guppies, platys, mollies, and swordtails. Males are often more colorful and smaller than females. They also possess external genitalia, having a gonopodium, which makes it easy to distinguish males from females: In males, the anal fin is rod-shaped, while females have a traditional fan-shaped anal fin. The gonopodium is used to fertilize the eggs inside the female as the male swims alongside the female. The fertilized eggs are carried inside the female and then give birth to fry.
Tetras
Tetras have some differences between the sexes, varying by species. Females are larger and slightly rounder than males. Males have longer fins and are often more brightly colored than females. Tetras are schooling fish, so breeding pairs can often be obtained by purchasing a small group of them at once. Generally, they breed by males chasing females and fertilizing the eggs as the female lays them on plants or scatters them among larger rocks. There is no parental care for the eggs, and they may be eaten if discovered by other fish.
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