How Do Fish Mate? (And How To Get Them To Breed In Your Aquarium)

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Have you ever wondered where baby fish come from? I mean, before you get them from the fish store that is. Most fish breed in a very different way from humans and land animals, and with over 30,000 species on the planet, these creatures have loads of fascinating ways of reproducing!

Ready to discover more about the weird and wonderful world of fish reproduction? Let’s learn and answer the question – how do fish mate!

Key Takeaways

  • Some fish are Hermaphrodites like clownfish and can change from female to male or male to female!
  • There are internal fertilization and external fertilization fish
  • Some fish are monogamous their entire life, while others have breeding seasons, others only breed once in their life
  • Aquarium fish breed with the following methods in aquariums: egg scatters, egg depositors, egg buriers, mouthbrooders, and livebearers

Fertilization – The Crucial Step

Before we get into the various breeding strategies of our fish friends, let’s start with a basic reminder on the concept of sexual reproduction.

The important thing to understand is that for a new individual fish to come into existence, it needs a sperm cell from its father to fertilize an egg cell from its mother. These cells fuse and multiply to grow into a complete little fish, ready to grow up and repeat the miracle of reproduction!

So where do fish eggs and sperm come from?

Reproductive Organs

Adult fish produce eggs and sperm, and different species reach sexual maturity at different ages. The Female reproductive organs are known as ovaries, and that’s where eggs (also known as roe) are produced. Male fish produce sperm (also known as milt) in their testes.

As you’ll learn later on, most fish simply deposit their eggs and sperm into the water, but some males have sex organs for depositing sperm inside the female. These organs are known as claspers or gonopodia, depending on the type of fish.

Did You Know? – Hermaphrodites

Melanurus Wrasse in Reef Tank

As strange as it sounds, some fish can be both a male and a female at different times of their lives! Clownfish start their lives as males and then change into females when they reach a certain age. Wrasse fish do just the opposite and change from females to males.

Mating Strategies

So, how do male and female fish bring sperm cells and egg cells together in the first place? Well, that’s where things get very interesting.

Fish come in all sorts of shapes and sizes, and they live everywhere from the deep dark ocean floor to shallow, fast-flowing rapids and even temporary puddles. Clearly, the same approach isn’t going to work for every species, and that’s why we see the amazing variety of reproductive strategies in the fish world.

Let’s take a look at some of the ways that fish eggs are fertilized.

Internal Fertilization

The first way that fish can reproduce is through internal fertilization. In this strategy, the male must deliver his sperm cells into the female’s body where they can fertilize her eggs. Live-bearing fish species like guppies and mollies reproduce this way, and so do stingrays and sharks.

Sailfin Molly in Aquarium

Internal fertilization can be tricky in water, and it puts fish at increased risk of predation while they mate. However, it also provides a higher chance of successful fertilization, although fish that breed this way tend to produce fewer offspring.

So how do fish mate underwater? They can’t exactly hold onto each other while they mate, so the males of many shark species literally bite their partner to stay in position. Livebearers like guppies do things a little quicker, and the male simply jabs the female with his gonopodium for a moment to transfer his sperm.

Species that practice internal fertilization are known as ovoviviparous and viviparous fish. Let’s take a look at the differences and similarities between these breeding strategies.


In ovoviviparous fish, the eggs are fertilized inside the female’s body and not released into the water. The baby fish then grows and develops by feeding on the nutrients stored inside its egg.

Ovoviviparous fish produce fewer eggs than egg layers, but their eggs develop in the safety of their mother’s body where they have a much higher chance of developing and hatching safely.


Here, the baby develops inside the female fish and gets all the nutrients it needs directly from its mother, kind of like we do. In some cases, however, the mother does not provide these nutrients, and the baby fish must eat each other or other tissues to grow. Brutal right?

Strange but True – The Curious Case of the Seahorse

Seahorse Couple

The seahorse does things a little differently. In a strange reversal, the female lays her eggs inside the male’s pouch, and he then fertilizes them and looks after the eggs until the little ones swim free.

External Fertilization

External fertilization is where fish release their eggs and sperm into the water to fuse outside of the female’s body. This common strategy is known as oviparity, and most fish (including most aquarium fish) reproduce this way.

This breeding strategy has some important benefits for female fish because it leaves them free to swim and feed without carrying so many eggs or babies. Of course, external fertilization is more of a gamble, but oviparous fish often produce huge numbers of eggs to improve their chances.


Some fish can have babies without fertilization at all. Their eggs develop without fusing with sperm, and the baby fish is a clone of its mother. This reproduction strategy is risky because the lack of genetic diversity does not weed out bad mutations.

So now you know the basic concepts of how fish fertilize their eggs. Let’s take a closer look at what happens next in the reproductive cycle.

Embryo Development

Female fish that practice external fertilization deposit their eggs in different places. They may release their eggs out into the open water where they drift for long distances on sea currents or deposit them in a specific place. Some fish lay sticky eggs that stay put on plants until they hatch.

Either way, the embryo in the fertilized egg will now develop until it is ready to hatch out as a larva or juvenile. This process can take just a few days for many fish, but shark eggs can take as long as nine months to hatch!

What Happens When the Eggs Hatch?

Baby fish hatch out at various stages of development depending on their species. The tiny babies of bony fish are called larvae, and they cannot swim freely. At first, they live off their yolk sac, which provides all the nutrients they need until they are ready to start feeding themselves. Below is a cool video from Chrisfix showing eggs hatching under a microscope!

Baby sharks are much better developed when they hatch. These cartilaginous fish skip the larval stage and hatch out as juveniles that look like miniature versions of their parents.

Ovoviviparous and viviparous fish that give birth to live young have relatively few babies, but they are larger and better developed when they are born. Great White Sharks have just 2 to 12 babies at a time. Compare that with the ocean sunfish that can lay a jaw-dropping 300 million eggs at a time!

How Do Fish Find a Mate?

Many fish live in the same general area throughout their lives, so finding a partner and choosing where to mate is pretty straightforward. However, for some species, mating involves amazing journeys and migrations.

Adult salmon live in the ocean but spawn in shallow, oxygen-rich streams far from the coast. These amazing fish return to the same stream where they hatched and swim far up, climbing waterfalls and dodging dangers like hungry bears only to mate and then die. Fish like salmon that mate only once in their lives are called semelparous.

Freshwater eels also migrate before breeding, but they do things a little differently. These fascinating creatures spend their adult lives in freshwater but travel thousands of miles to lay their eggs out at sea. You can check out how crazy eel reproduction is in our video below from our YouTube Channel.

Fish that spawn in large groups need to know where and when to meet each other for the main event. These fish often congregate in the same areas and at the same tide and moon phase each time.


External fertilization is pretty impersonal. In some cases, large schools of fish simply come together and release their eggs and sperm in the open water. There’s no time for being picky; these fish breed in bulk!

Large schools of spawning fish attract loads of hungry scavengers looking for a free fish-egg breakfast, but with so many eggs in the water, some will always float away unnoticed.


However, some fish have more intimate monogamous relationships, and many species go through complex courtship rituals to choose a partner, build a nest for their eggs, and even care for their young.

Monogamous fish form pair bonds where a single male and a single female fish mate and reproduce. This breeding strategy is actually pretty rare, but seahorses and freshwater cichlids are good examples. Mouthbrooding cichlids even protect their young by allowing them to shelter in their large mouths, safe from predators.

The Siamese fighting fish or betta is another well-known example of a monogamous fish. In a fascinating mating ritual, male bettas fertilize their partner’s eggs as they are released and then place them in a bubble nest at the water’s surface.

Sexual Maturity

Fish are ready to mate when they reach sexual maturity and their reproductive organs are developed and fully functional. The age when fish mature varies tremendously between different fish species depending on their lifespan and biology.

Nothobranchius furzeri, the beautiful turquoise killifish of Africa, is sexually mature at just two weeks old. These small freshwater fish live fast and die young, completing their entire lifecycle in less than a year! They have such a short lifespan because they live and breed in temporary rain pools.

Nothobranchius furzeri

Whale sharks, on the other hand, have all the time in the world. These magnificent beasts live life in the slow lane and are first able to breed when they are 25 to 30 years old.

Breeding Season

Many fish species mate at a specific time of the year, although this varies. Salmon, for example, spawn in the fall, while most fish species breed in the spring. Temperature and day length changes tell the fish when it’s time to breed.

Most fish are iteroparous, which means they can breed several times in their lives. Others, like the blood-sucking lamprey, are semelparous and breed just once in their lives.

How Do Fish Mate in Aquariums?

Up until now, we’ve covered some of the basics of fish breeding theory and hopefully answered the question of how do fish mate.

On a more practical level, understanding the basics of fish reproduction is important for the more advanced fish keeper who wants to breed fish as a hobby or for extra income. So how do fish mate in aquariums?

Let’s take a look at some basic concepts for the beginner fish breeder.

Aquarium Breeding

Aquarium fish species use many different breeding strategies, but most are egg layers (oviparous). We can go a step further and divide most popular species into egg scatterers, egg depositors, mouth brooders, and livebearers – here’s what you need to know:

Egg Scatterers

Egg scatterers simply scatter their eggs over any surface and ‘hope for the best’. These fish lay large amounts of small eggs that hatch pretty quickly, and they do not bother to protect them from other fish and predators.

Egg Scattering fish examples:

Egg Depositors

Red Betta Fish

Egg depositors take a little more care and lay their eggs in a nest, cave, or other specific areas where the male can fertilize them directly. These fish generally lay smaller batches of larger eggs, and often protect them from other fish.

Egg depositing fish examples:

Egg Buriers

Some fish bury their eggs in the substrate. Annual killifish are a great example of egg buriers, and they do this to keep their eggs moist in the dry season when their puddle evaporates.


Some species of fish take caring for their eggs to a whole other level. Instead of leaving their eggs to drift, or depositing them in a nest, mouthbrooders keep their fertilized eggs safely behind closed jaws!

Mouth-brooding fish examples:

  • Sea catfish
  • Cardinal fish
  • Discus


Livebearing fish are the easiest species of fish to breed in the aquarium because they do not lay eggs. These fish become visibly pregnant and give birth to free-swimming babies. Livebearer babies are tiny, but they are better developed than the fry of egg layers and easier to care for.

Live-bearing fish examples:

Aquarium Breeding – Basic Steps for Beginners

Naturally, you will need at least one male and female of a particular species to breed successfully, but putting fish in a glass box and hoping for the best rarely produces results. There are a few things you can do to improve the chance of successful mating.

Let’s take a look at some basic steps that apply to breeding many species of fish.

Research, Research, Research

Each species of fish has different breeding behaviors and requirements. You’ll need to do your homework on the most reliable breeding techniques of your chosen fish species before you get started.

Some popular aquarium fish have never been bred in captivity or need special hormone treatments to induce spawning. Choose easy species like livebearers or hardy egg-laying fish like danios if you’re just getting started.


Fish need to be in peak physical condition to reproduce. Breeding takes a lot of energy out of male and female fish, so supplementing their diet with live and frozen foods in the weeks before breeding is highly recommended.

Get Them in the Mood

You can encourage your aquarium fish to breed at any time of the year by making gradual adjustments with your heater and aquarium lighting. Increasing the temperature and photoperiod gradually makes them think that spring has sprung!

Provide a Safe Place

Many fish keepers prefer to keep a variety of species in their aquariums, but most fish see eggs as a tasty snack, so it’s best to move your breeding fish into a separate tank for the safety of the eggs and fry. Bear in mind that many fish will even eat their own eggs and babies, so you’ll probably want to remove the parents after spawning.

Nest builders like cichlids can become highly territorial and protective over their eggs, so it is often safer for your other fish to breed them in their own tank.

Caring For Babies

If you successfully breed your fish, the next challenge is caring for all the little babies, and feeding fry can be a tricky task!

Fish fry are too small to eat regular flake food or pellets, so it’s best to prepare ahead of time and grow some infusoria for them. Powdered food and newly hatched brine shrimp are ideal food sources once they grow a little bigger.

Finding Homes

Fish can produce an impressive amount of babies, so it’s important to plan ahead and make sure you’ll be able to find a good home for all the little ones! Your local fish store might be interested in selling them for you, or you can give them away to other fish-keeping friends.


How do I know if my fish are mating?

Fish often mate in aquariums without us even knowing. You might spot some tiny fish fry hiding out somewhere in the tank, or you might never know your fish even laid eggs because their tank mates gobbled them up!

The best way to know if your fish are breeding is to observe their behavior. Each fish species has different mating behaviors, but common signs include nest building, increased aggression towards other fish, or guarding a certain area. They may also be following each other constantly or swimming side by side in a quiet part of the tank.

How does a male fish impregnate a female?

Most female fish do not get pregnant. Instead, they lay their eggs and the male fertilizes them externally, out in the water.

However, some fish species do practice internal fertilization, and the process is similar to the way mammals like us mate. Male guppies, for example, have a modified anal fin called a gonopodium that they insert into the female and release their sperm.

What is the mating process of fish?

Fish reproduce in many different ways. In most species, males and females come together at certain times of the year to spawn. The female will release all her eggs which either fall into a nest, drift in the water, or stick to rocks or vegetation.

The male fish will release his sperm over the eggs, or into the water column at the same time to fertilize them. Most parent fish leave the eggs to hatch and the babies to fend for themselves, but some fish species protect their eggs and babies until they are ready to fend for themselves.

How long do fish mate for?

Fish mating can take a single second or several hours, it all depends on the type of fish. The male guppy can impregnate the female in the blink of an eye, but a male and female betta have a long mating process that involves building a nest, embracing, egg-laying, and placing the fertilized eggs in the nest.

Final Thoughts

Many fishkeepers and nature lovers find themselves asking just how do fish mate? Whether you are trying to breed a particular fish species, or you’re just interested in learning more about these fascinating creatures, this article should clear up some of the secrets of fish mating!

Have you ever seen your fish mate? Tell us about your experiences in the comments below!

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