Many people are interested in finding out the gender of their goldfish. You might want to know for breeding purposes, or just to make sure you didn’t name your female goldfish George. Luckily, you can identify whether your fish is a male or a female by paying attention to the fish’s size, shape, appearance, and behavior. Although not all fish can be easily identified, the following tips can be very helpful for you to recognize the gender of many common aquarium fish.

In General

1. Compare the size of the fish during spawning season.

With many fish, like koi, females tend to be larger than the males. This is especially true during spawning time, when females have large abdomens where they carry eggs( Once the eggs are laid during spawning season, the females and males can return to the same size). To check the size of a fish in the same species, you can look at them when they’re near each other, take a picture of each one, or catch one in a net to quickly use a ruler to measure its length and width.

2. Look for a bump on the forehead of male fish

Look carefully at the face of the fish just between the eyes and above the mouth. If it has a large, protruding bump, there’s a good chance that the fish is a male. This bump is called a “nuchal hump,” and it’s present on many types of fish, like the tilapia, angelfish, oscar, and discus.

3. Watch for changes during mating season.

Male fish tend to grow bristles, bumps, or shiny scales during breeding season to attract females. Females can sometimes also grow these attributes, but the male versions tend to be larger and more pronounced to attract attention

Specific Species

1. Male cichlid fish have bigger dorsal fins 

In general, male cichlid fish have larger dorsal fins on their backs than their female counterparts. If you have multiple cichlids, look at the size of their fins when they are next to each other to identify which is the male and which is the female.

2. Male goldfish often have seasonal bumps

During the springtime, look at the forehead, gills, and pectoral fins of your fish. If you see small clusters of bumps in these areas, your fish is most likely a male goldfish. Don’t wait too long, however, because these bumps are only present during the springtime, and will disappear when goldfish breeding season is over in the ear summer.

3.Male pleco fish are more aggressive

Male pleco fish can exhibit aggressive behavior toward other fish. Keep an eye out for plecos that swim quickly toward other fish that come near them to scare them off. If you have an aggressive pleco, it’s most likely a male.

4. Male guppies and mollies have triangular fins near the tail

Toward the back of the fish, near the tail fin, you might be able to see a small fin in the shape of a triangle. This is called an “anal fin” and can be used for sexual reproduction. It might be hard to see this fin on smaller fish, so get your magnifying glass out if you have to!

If you tried all the mentioned tips but still could not identify your fish, it might be time to pay a visit at local pet store and have professionals answer the question for you.