How to Breed Koi Fish

Are you thinking of expanding your koi population? Do you want to start koi fish breeding? If yes, it’s a good idea because it can be both a hobby and a profession. You can begin as long as you have the necessary time, space, and budget for it. 

Koi fish may reproduce on their own in large size ponds. However, there is the risk of fish eating their baby eggs and baby koi fish. 

There are a few steps you will have to take for their safe and formal breeding if you want to add to your koi population. 

Read on to find out about the different steps involved in koi fish breeding. It includes things like the right time for spawning, how to select the right partners, and how long it takes for koi to reproduce. 

Spawning Koi Fish

How to Breed Koi Fish

Breeding koi fish isn’t so difficult, as long as you follow the right instructions, and have the time and patience for it. Remember, you are breeding fish, so you cannot expect overnight magic. The results will, however, be worth your patience, time, and perseverance! 

Here is stop by step instructions on how to breed koi fish. 

  1. Prepare the spawning pond for koi fish breeding.
  2. Select the parents
  3. Identify male and female koi fish.
  4. Prepare the ideal, spawning environment.
  5. Help koi fish lay eggs
  6. Take care of baby koi fish
  7. Cull fish
  8. Reintroduce parent koi fish

How To Prepare A Spawning Pond For Koi Fish Breeding

You need to select the right sized pond for breeding. The spawning pond should ideally be about 6 to 12 sq. meters big. Clean and fill it with unchlorinated water to a depth of 50cm. You’ll need a bigger pond if you have more fish.

Clean Pond Water

Do entirely clean out the pond if it’s dirty or filled with algae. Remove any debris, predators, and anything that may hurt the koi from the pond using a skimmer. 

You must keep the pond water clean using a specialized koi pond filtration system. Though the system is expensive, the water should always be clean for koi breeding and the fish and spawn health. 

Sufficient Spawning Material

The pond should also have enough spawning material to encourage the female to lay eggs on it. You can either make your mat or buy one from a pet store or specialty fish shop.

Place the flat and sticky fry mat somewhere apparent in the pond to give the koi place to lay eggs. It’s important because they may not mate if they don’t find any place to lay eggs. 

Spawning Rope

Many modern hobbyists prefer using synthetic spawning ropes as spawning material. It’s because they are free of parasites, don’t get easily damaged, and it’s easier to handle eggs than in natural spawning material. 

Many old-school breeders prefer using natural spawning material. However, it’s better to use synthetic versions if you plan to move the eggs away from the parents once spawning is complete. 

Cover The Pond

Keep the pond covered with a net and secure it down using massive rocks if you have cats or birds nearby that may approach it. 

If you plan to breed a bunch of fish, then it’s better to prepare a separate tank for the adult koi. It’s to prevent them from eating the hatched eggs. As usual, the tank should be clean, with a filter system and preferably 100 gallons capacity to hold two koi. 

How To Select The Parents

You have to start koi fish breeding by first selecting the right koi. You need to choose fish that are at least 3years old and measuring about 10 inches or 25 cm long.

This is very important because koi sexually matures only around the age of three. 

According to experts, the best time for koi breeding is when the parents are 2-4years old. It’s not difficult to select them, but it can be tricky. 

Look for healthy koi without any disabilities. The parents should have excellent body conformation, colors, and markings. Choose the parents based on the types of baby koi you want.

Be aware that not all koi varieties give nice koi when bred together. You need to do your research on koi variety pairing before breeding them. 

It means that if you want Kohaku fry, you need to select parents who are both Kohaku.

How to identify male and female koi fish 

You also need first to know which koi are female, and which are male. 

It’s difficult identifying the fish’s sex in koi that’s under a foot long. However, you can also identify them by comparing their size. Male koi are usually smaller and slimmer than their female breeding counterparts but are larger than females when young. 

The female koi fish are much more round than male koi, especially when they are about to lay eggs. Male koi are easy to spot because they tend to chase females around the pond when they start breeding.

Male vs Female Koi Fish
Image Source: Dexters World

Different Physical Attributes

Larger female koi are more likely to endure the breeding process than their smaller counterparts. They also produce larger eggs and healthier baby koi that grow up quicker than the fry produced by smaller females. 

Female koi can carry thousands of delicate, translucent eggs and have a bloated appearance that’s usually one-sided, and towards the left side. However, bloating in immature female koi or later in the mating season usually indicates disease or some parasite. 

The female koi has broad shoulders with a fuller body. Men koi have more pointed and larger pectoral fins than round female fins. 

You can also tell the koi’s gender by taking a look at the vent on its underside. While both male and female koi fish have a line from head to tail, the females have another crossing in a ‘T’ shape. Besides, the female’s vent is rounder and pinker than a male’s. 

Next, touch the koi’s pelvic bones. You will notice that females have a soft abdomen to hold eggs in between their bones. You will also see that the bones are fused. However, all this is noticeable only in mature fish and not young ones. 

As mentioned earlier, female koi start carrying eggs only when they are three years old, when they are fully mature. While younger koi can breed, they produce low-quality offsprings. 

Different Eating Habits

Feeding Koi Fish

You can also distinguish male and female koi through their eating habits. Female koi are naturally aggressive while feeding, and tend to stay continuously eating near the surface. Male fish tend to swim around throughout the eating session. 

You will also see that male koi fish develop tiny, rough white spots called tubercles on their face sides during the spawning season. Females don’t have tubercles. 

It’s okay to have many male koi in the same pond.

How many male koi in a pond?

But the koi’s breeding act is physical and can significantly harm the females if the males are aggressive. While some breeders may use two males for a single female koi to maximize their yield of propagation, the 2:1 ratio shouldn’t be exceeded. 

However some breeders prefer using only one male because of the higher predictability of the type of offspring it offers. if this is the case, it’s better to keep a few back-up males in case one gets too aggressive. 

However, do get rid of koi you don’t want to breed to prevent ending up with unwanted baby koi. Select and retain only koi fish with the attributes you wish to see in the baby koi. 

How to prepare the spawning environment

Once you have selected the prospective parents, isolate them in a separate pond to condition them for spawning. Most breeders isolate the parents at least a month before the anticipated spawning date. 

Males are separated to prevent indiscriminate spawning.

Feeding your koi fish four times a day keeps them well fed. It proves advantageous to you because well-fed koi fish have a better chance of breeding. Feed them as much healthy food options like wholemeal bread, oranges, and lettuce.

Don’t overfeed them. Food which they can eat within 5 minutes is more than enough. If you find food lingering on the water surface after 5 minutes, it means you are overfeeding.  

Increasing their protein intake also makes their bodies stronger, which is a plus point for breeding. You can get protein supplements at pet stores. 

The female eventually gets rounder noticeably and bloated with eggs. She gets heavy with a soft abdomen, signalling when she is ready for spawning.

The shift

It is at this point that she’s carefully shifted to the spawning pond. Make sure she’s constantly supported by water while moving even if she’s carried in a net. 

The male is also presumed to be ready for spawning and can be shifted to the spawning pond. Make the shift a few hours after the female is accustomed to the spawning pond.

Transporting Koi Fish

The actual spawning process

The female starts releasing pheromones once the male enters, and they will breed overnight. 

That’s why most breeders introduce the male in the evening. It’s perfect because spawning usually takes place in the wee morning hours.

The male tends to wrestle the female aggressively by head-butting, shoving, and biting her while spawning. He does this to push her towards the breeding foliage. 

It’s during the wrestling match that the female koi releases eggs that get attached to breeding foliage. It’s while the eggs are on the foliage that the male’s released milt or sperms fertilizes them. 

The milt then enters and fertilizes the eggs.

The same process repeats until the female gets tired and stops responding. This is when she is removed and placed in the nursery tank while the male goes to the pond. 

Be patient

Last but not least, be patient and let the koi mate. You never know when they will mate. There’s no guarantee for spawning just by placing male and female koi together in a koi pond. It’s because koi fish prefer to spawn in the pond’s shallow areas.

Spawning can be immediate or in a few weeks. They may even seem uninterested in each other at first. 

There is nothing to worry if this happens because it’s a natural phenomenon Nature will take its course, and a full moon, storm, or air temperature change will encourage them to mate.

You will know when the koi mate. 

Froth appearing at the water surface layer indicates that the eggs have been laid. Take a look in the water now. You will be able to see their eggs on the spawning mat. This is when you should remove the parents from the pond if you are breeding koi for profit. 

It’s because there is always the chance of the parents eating some eggs.

How do koi fish lay eggs?

Koi like to hatch their eggs on a substrate, pond plants, spawning mats, or spawning ropes. They may also lay their eggs on twigs placed on the pond floor. 

Wherever they do lay their eggs, you must watch them carefully. Many koi may end up snacking on them. So it’s better to place hungry koi in another pond or tank and protect your hatching eggs. 

Koi eggs are tiny and bead-like specks that vary from a milky white to yellow or green. Close inspection will reveal two black dots through the egg’s transparent membrane. These are the koi’s developing eyes. 

The eggs typically hatch in less than a week, and the small koi fish are called koi fry. The baby koi attach themselves to the pond side walls and learn to swim within a few days. They live off the nutrients of their egg yolk in the first few days and later depend on you for food. 

How to take care of young koi

Make sure you have a protected area ready for the baby koi fish when they first hatch. Remember, they can’t swim immediately and are vulnerable to predators.

You can use a fine mesh to protect and prevent them from getting sucked into the fish pond filter. 

Koi fry have tiny mouths, and need smaller sized food. They eat liquid or little suspended particle food for the first few days, and you can later increase the koi food size. 

You can start feeding the baby fry koi pellets in the powder form when thety are ten days old. Just crush the pellet to a fine powder and sprinkle onto the water.

You can also feed baby koi regular food like hard-boiled eggs and even commercial foods like baby brine shrimp.

Feed them four times a day, and pour enough food to last them 5 minutes till they are four weeks old.

You can help them grow by ensuring there’s enough aquatic plant life coverage that protects them from predators. This is essential because koi fry are vulnerable at this point. They need to be protected under the cover of pond plants from outside predators like birds and raccoons, and inside fish. 

Culling fish- why you should do it

You will have to cull the fish if you are breeding koi for profit or if you have a small pond. It’s because there’s not enough space, and because not all koi fish are ideal for breeding.

The koi’s colors and patterns are a genetic mutation, which is why only a small percentage of the koi offsprings are attractive enough to live and have market value.

Koi Colors

In most cases, 98% of the offsprings are unattractive without any commercial value. And only about 2% of less of them are considered potential or ‘keepers’ and are selected from the spawn to grow further. 

That’s why koi crops are culled throughout the year. In short, culling involves identifying the very small koi with some physical problems or without the desired color patterns. 

The first culling occurs in mid-summer where groups of ‘pickers’ spend hours searching for koi with signs of growth potential, and cull the others.

The potential ‘keepers’ are left to grow and then culled repeatedly as they grow. 

The fish are removed, sent to quarantine, and sold while the best grow further. It’s the best of the best that grow into premium select grade koi measuring 10-16 inches long. 

While you can cull koi whenever you feel right, it’s better to wait till you can see a pattern on their bodies. You can then decide which koi you like or have your favorite colors, and you want to keep. 

Whenever you do cull them, do it humanely. Besides, you don’t always have to dispose of them. You can always give a healthy koi to family and friends. 


1. How to breed koi fish in a pond

You will have to first have the right sized pond for your koi fish. Then place spawning mats on the edges of the ponds to let the koi fish spawn naturally. Remove the mats once you see signs that the koi have stopped spawning. 

Don’t forget to keep monitoring the pond to ensure it’s the right environment for the koi fish fry and parents. Also keep checking the pond for overcrowding, its oxygen levels, and remove any unnecessary weeds, bugs, and predators.

2. Is it hard to breed koi fish?

Nothing is easy in life.

However, with the right perseverance, tips, and knowledge, you can indeed breed koi fish. It’s up to you to decide if you want to produce fish for hobby reasons or a little side business. 

3. How big do koi need to be to breed?

You need to wait till the koi fish are about three years old to start breeding them. It’s because that’s when they reach sexual maturity. Koi fish are usually about 10 inches or 25 cm long once they reach 3 years old. 

4. How long is a koi fish pregnant? 

Like most fish, koi fish do not get pregnant. They reproduce through the spawning process. This is when their bodies get filled with eggs and they then lay them at the right time.

Koi fish start carrying eggs around wintertime for a few months until the mating season. It’s interesting to know that koi fish have a short mating period lasting only two to three days.

It is during the mating period that the female koi fish lays eggs. She lays it on various surfaces, and one or two males soon fertilize them.

5. How many babies do koi fish have at a time? 

Female koi carry several thousands of delicate, translucent eggs at a time. However, they take about a year for the eggs to develop fully in their body. It means that the eggs formed in the first year’s spring drop in the following year’s spring. 

The eggs are not fertile. The male koi has to release sperms into the dropped eggs. It’s when spawning takes place that new eggs start forming, and are released in the next year’s spawning. 

6. What are Koi fish breeding mats?

Koi fish don’t just lay their eggs anywhere. They need to lay it on breeding or spawning or fry mats. You can buy these mats at the pet store or make some with non-toxic, hardy aquatic plants.

You can make one using a plant that is thick, about 4 inches deep. Make sure you buy it well before spawning season. It gives you time to treat it for parasites in a quarantine tank. 

You can also use tightly packed Spanish Moss or frayed nylon rope to create mats. You can use anything that’s nontoxic and 4 inches deep, so that it’s thick enough to catch the eggs. 

Also follow the instructions on a commercially available plant pesticide to find out how much you have to apply to the tank’s water to ready the pond for spawning.

Also find out how long you need to quarantine the spawning plant so that it’s not dangerous to the fish. 

7. What are the best koi fish breeding time and season?

The best breeding time is during warm weather when the temperature has risen. That’s when the female koi can produce as much as 1 million eggs. 

8. When can you reintroduce the parent koi?

You can safely reintroduce the parent koi into the pond with the baby koi once the babies are about 3 inches or 7.6 cm long. The parents become friendly with them once they grow bigger. Baby koi usually need three months to reach 3 inches long. 

9. How come my koi aren’t breeding?

There are many reasons for this. The first may be because they are too young. Remember, females need to be at least 3 years old, and males at least 2 years old to spawn.

Koi fish that are not nutritionally strong enough do not spawn. Female koi fish need to consume lots of proteins and live foods to become strong enough to spawn with males.

Sometimes they don’t breed just because they don’t have their privacy! Yes, they need their privacy! So give them their privacy by separating your chosen female and two males in the spawning pond for some time. Make sure you have created the right spawning environment that promotes breeding like clear, freshwater, and spawning media.

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