How to Determine the Sex of Your Fish

Most aquarists would like to know the gender of the fish they have, and not just for breeding, but for choosing tank mates as well. But finding out the gender of your fish is sometimes a frustrating task, and impossible to determine visually in some species.

Because of that, the simplest way to find out the gender of your fish is to do research on the species you have. However, there are some signs that you can use to identify the gender of your fish. With a few tips, you might be able to determine if your fish is female or male in various species.

Best Ways to Determine the Sex of Your Fish

The best way to determine the gender of your fish is to compare their size during the spawning season. Many of the female fish are usually larger than the males, especially during spawning when females get large abdomens. To check and compare the size of a fish within the same species, you will need to look at them when they’re next to each other. Once they are next to each other, you can take a picture of each one, or if you want to be sure you can catch them to quickly measure their length and width with a ruler.

You can also try looking for a bump on the fish’s forehead. If there is a bump above the mouth and between the eyes, it’s a male. The bump on the males is called a nuchal hump, and it can be seen on different species like oscar, angelfish, and discus. However, some species do not have this nuchal hump, but if the fish has one, it is a sign that it is a male.

There are also several other bumps of various shapes, sizes, and colors that can help you identify a male fish. In the spring, there can be small clusters of bumps around the pectoral fins, gills, and forehead. However, these bums disappear in the summer, so make sure you notice them while they’re there.

Goldfish

Apart from having the easiest care, goldfish are very specific, and easy to determine which gender they are. The best way to determine differences between male and female goldfish is to look at their physical characteristics. Females will have some fat around their bellies, and not just when a goldfish is pregnant, while males will have roughened pectoral fins along with a few raised bumps on their operculum.

Angelfish

With angelfish, it can be very difficult to determine the gender of your fish, especially while they’re young. Male fish can occasionally have a modest nuchal hump above their eyes, which can help you distinguish males from females.

Along with their physical difference, angelfish will pair off to mate once they are mature enough. To properly mate your fish, you can get a half-dozen immature angelfish, raise them together, and you will definitely get at least one breeding pair. Once they start mating, you will be able to notice which fish is female and which one is male, since the female will be laying the eggs.

Bettas

Bettas are pretty easy for determining the gender of each fish so there’s not much to worry about with betta fish care. Males will have long flowing fins with brilliant and attractive colors. Because of these colors, pet shops will have mostly male fish that people will buy. Females have much less color, along with short and stubbier fins. However, with these species, it’s important to separate males from each other, so that the betta fish don’t get into a fight.

Tetras

The difference between male and female tetras is based on the species, but there are some characteristics to help you.  The female fish will be slightly larger and plumper than the male fish. Apart from that, just like bettas, males will have more vibrant colors and longer fins.

Koi

Male koi are skinnier and slightly smaller, while female koi are larger and rounder. While it’s spawning season, the vent of a female koi will be puffed out and swollen, while the male will remain tucked in.

Cichlids

Cichlids are a very diverse group that has a lot of differences within each species. But there are some physical characteristics between males and females that can help you tell the difference. Males are slimmer, vibrantly colored, and have larger bodies. Along with that, males will have a nuchal hump on the head. And the male will also display egg-shaped markings called egg spots on its anal fin.

Cyprinids

The fish in the cyprinid family, like barbs, can be very difficult to tell apart. The differences between males and females will vary with species, but males are usually colored with more intense and vivid colors than females. Along with that, males will be slimmer with small white bumps on their heads. And apart from the physical characteristics, you can see males chase the females during mating season while the female fish lay eggs and let the male fertilize them.

Gourami

Gouramis are not easy to determine between genders since females and males are similarly shaped and colored. However, there is one difference that can be seen in most gourami species – the dorsal fin. Males have a long dorsal fin, while the dorsal fin of females is shorter and rounder.

There are some species of gourami fish that have color variations for males and females. With these species, the male gourami will have a deep red-orange coloration around the breast and throat or orange to red coloration around the pelvic fins.

Guppies

As live-bearing fish, guppies can be a big problem if the males and females cannot be separated. But luckily, it’s not that difficult to tell them apart. Apart from being larger than males, the female guppies have rounder bellies and smaller, triangular-shaped anal fins, while male fish have long and pointed anal fins.

Oscars

Oscars are aggressive freshwater fish that can be difficult to distinguish between males and females. They have no color difference like other fish species. The best way to tell between male and female Oscars to observe them closely. You should especially observe them once they start laying eggs to find the female oscar. You can also watch the underbelly of the Oscars and look for a small spike or thorn-like protrusion to find the male.

Sources:

1. How Do You Tell If a Fish Is Male or Female? – Cuteness
2. How to Tell the Gender of a Fish – Aquatic Veterinary Services