Skip to main content

Easy tricks to learn the sex of your fish

Whether you’re interested in breeding or just want to choose an appropriate name, knowing how to tell the sex of your fish can prove highly useful. The determining features that identify fish as male or female are different for each breed. However, there are some common ways that fish parents can determine the sex of their fish.

how to tell fish gender
Image used with permission by copyright holder

General tips and tricks

Most gender characteristics will not show themselves until the fish have reached their sexual maturity. This age is different for many breeds, but it is safe to say that around 2–3 months old, most fish species are sexually mature. Meaning any colorations, unique gender growths, or spawning signs will be more apparent and more easily noticed.


As with many birds, male fish tend to be more colorful. This color difference between males and females can exist for multiple reasons. Males are more colorful to attract a mate or to distract predators from their eggs. Male betta fish even chase the females away from the eggs.

Head bulges

Many male fish like tilapia or oscars will grow a distinctive bump on their forehead. Located between the eyes and above the mouth, this bump can make it easy to identify a male fish.

Growths and designs

During the spawning season, some male fish will develop impressive growths or designs. These identifying characteristics will appear anywhere on the body, including the head. For example, male pleco fish will grow long whiskers on the edge of their mouth and pectoral fins.

Spawning season differences

Spawning season is usually the best time to identify the sex of your fish. During this time, females will appear larger and rounder than their male counterparts. The surest way to know the sex of your fish is to wait and see who lays the eggs. However, if breeding isn’t a part of your fish-raising plans, you might want another option.

Aggressive behavior

As fish attain sexual maturity, many male fish will become aggressive toward other fish in the tank. These hostile manners can be observed when other fish approach them or swim into what the aggressive fish has decided is his territory. The territorial fish will charge the intruder. If you see this behavior in your tank, you most likely have a male.

Specific species identifiers

Although the tips above can help with many breeds, some fish have a unique way of showing gender. Of course, many fish are not discussed here, but many common or popular fish species will be. For more rare and exotic species, be sure to contact an expert for advice.


In season, male goldfish will develop bumps around their gills, foreheads, and pectoral fins. However, these bumps are there only during the breeding season. Once spring has ended, you’re not likely to find these bumps.

Betta fish

The picture of a betta fish is typically a beautifully colored fish with large, flowing fins. That is a male betta. Possibly one of the easiest fish species for identifying the sex, bettas are brightly colored when male and usually dull brown with smaller fins when female.

Guppies and mollies

For guppies and mollies, it’s easiest to look for the anal fin. Toward the back of the fish, near the tail, there will be a triangular fin called an anal fin. This three-sided appendage is present only on males and is used for reproductive purposes. Keep in mind that it might be harder to see on a smaller fish.


Pleco fish

As mentioned above, male plecos will grow long whiskers that are hard to miss. Females also grow these adorable whiskers, but they are significantly smaller and shorter than the males’. If it’s hard to determine whose whiskers are longer, watch for aggressive behavior. Male pleco fish often charge other fish that get close.

Cichlid fish

Some common cichlid fish are tilapia, angelfish, oscars, and discus. These can be some of the most challenging fish for identifying sex. Waiting until they are of mature age will help immensely. When a male cichlid is mature, its dorsal fin is noticeably larger and longer than the females’.


As with cichlids, identifying the sex of gourami fish is difficult. But you may be able to identify the sex by looking at the dorsal fin. In males, the dorsal fin is long and tapers to a point. Females have a shorter and more rounded dorsal fin.


Tetras have a mixture of previously talked-about signs. As with many fish, female tetras are usually longer and fuller in shape, and males are more colorful and have longer fins.


Another hard-to-sex fish is the koi. Even many experts make mistakes in sexing koi, and most shops and aquariums will use ultrasound to verify the sex of their koi. You can look for size differences. Females, once again, will be larger and rounder, while males will be skinnier.

Fish are some of the hardest pets for identifying their sex. However, with these tips and tricks, you’ll be able to try your hand at figuring out the sex of your fish.

Want more? Read on further to learn how to breed and raise minnows. Or if you want something else, learn how to train your betta fish with our top tips.

Editors' Recommendations

Rebecca Wolken
Former Digital Trends Contributor
Rebecca's has written for Bob Villa and a Cincinnati based remodeling company. When she's not writing about home remodeling…
Best reptile pets: These are the 5 most affectionate reptiles you can welcome into your home
These friendly reptiles will make great additions to your family
Basking Chinese water dragon

When you picture an adorable pet, you probably don't visualize an iguana. Reptiles aren't generally considered the cutest of animals, but that doesn't mean you can't find a cuddly one. Whether you're looking for a new buddy for yourself or for your lizard-obsessed kid, there's a reptilian beast out there that will work great in your home. With proper socialization, these guys can learn to be handled daily, some even by children. If you want a new pet that enjoys human company, consider one of these most affectionate slitherers — they're the best reptile pets for handling.
Are reptiles high maintenance?
Not necessarily. One of the things that makes reptiles tricky is how foreign some of their needs are. Parrots, dogs, and humans often have similar requirements for food, exercise, warmth, and water. Since we're all warm-blooded omnivores (for the most part), it feels natural to care for many of the animals we welcome into our homes.

Reptiles are totally different. You'll have to think carefully about heating lamps, cool spots, gut loading, moisture levels, and skin shedding. So you only want to go down this path if you feel ready. That being said, with the right mindset, many lizards, snakes, and turtles make solid beginner pets (a few species don't and we suggest holding off on those until you turn pro).

Read more
Check these 3 things immediately if you have fish swimming at the top of the tank
Here's what might be causing fish to swim on the top of their home
Fish swim around in a tank with a bubbler

Before you set up your first tank, you likely didn't realize how much work went into maintaining the perfect ecosystem. In nature, we have the checks and balances of evolution to guide the delicate balance, but in an aquarium, it's just you. Learning how to clean, feed, and decorate takes time and research but will certainly benefit you — and your swimmers — in the end. Since fish can't tell you what they feel or even bark to let you know they need something, you'll discover other cues that tell you something's up. If you find your fish swimming at the top of the tank, take action right away. Here's what to do when your fish spend too much time at the surface.
What does it mean when your fish swim to the top?
Usually, your fish go to the surface when they're not getting enough oxygen. Unlike you, fish use their gills to breathe the air that's already in the water. However, in some circumstances, there's not enough oxygen in the tank for all the fish to breathe easily. To combat this, they swim up to the surface, where there is oxygen-rich water. You may notice some gasping or other signs that they're not well while they are up there. 

How do you oxygenate a fish tank?
The basic solution is to keep your tank oxygen rich and also low in CO2. You can do this by moving the water a bit more with an air pump or fan, which exposes the liquid to the air and helps get it ready for easy breathing. Be careful not to negatively impact other factors like temperature (more on that in a bit). Whatever you decide, make sure you have a long-term solution in place or the fish will go right back to the top again.

Read more
What you need to know about sugar gliders before you get an exotic pet
Follow these steps to set your sugar glider up for success
Sugar glider clings to their owner's thumb

Choosing a small pet involves almost as much deliberation as selecting a breed of dog. While there are a lot of factors to take into account, a sugar glider might turn out to be the perfect fit with their curious personality, attachment to your family, and fondness for pockets.

Like any exotic pet, gliders require expert care plus some dedicated research to choose the right breeder or pet store. But with the right prep, your new mammal will fit in perfectly and bond with the whole family. Keep reading to find out if sugar gliders are good pets.
What are sugar gliders?
Unlike most little pets, sugar gliders aren't rodents but marsupials. This gives you a few distinct advantages, as they behave differently from hamsters, guinea pigs, or gerbils. For starters, these are highly social creatures and they will bond with every member of the family and even other pets in the house. Because gliders don't smell like the animals your cats and dogs like to chase — rats, gophers, and bunnies, to name a few — many bigger pets can get along with your new friend. You'll need to introduce them carefully, but they can form lifelong attachments to each other.

Read more