Home » Information » 04. Breeding Koi

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  The following is a short introduction to breeding Koi.

One problem from the start is Sexing Fish.and is always going to be a bit hit and miss even the experts have been known to get it wrong.

Some basics

Females, especially in season are far more rounded than male fish which tend to be slim at all times. The pectoral fins of male fish tend to be larger and more pointed than those of female fish, but this is by no means always the case. During the breeding season, male fish can display a rough surface to the gill plates, again however, not always. To be sure, net the chosen fish and apply gentle pressure along the lower abdomen wall with your thumb and forefinger towards the vent. Male fish show signs of milt from the vent, whereas females in season will provide eggs! This operation should however be performed with great care so as not to injure the fish.

Probably the only sure way to sex fish is to watch the fish. The ones that lay eggs are female, the ones that chase are the males !!!

Next step is Pairing

This is always initially a process of trial and error. Always pair like varieties if you are trying to produce a certain variety. For example, if you are trying to breed kohakus, use kohaku male and female fish. Whilst crossing a kohaku and sanke/showa is acceptable, crossing a kohaku and a Yamabuki for instance will probably produce only ghost and black koi. Remember that male genes tend to be dominant in any pairing. Always select adult fish only to ensure successful spawnings. Once you have found a pairing that gives some good fish ? keep it.


Separate males and females at least one month before you intend to spawn the fish. Provide a reasonably small pond ? ideal 2 x 3 x 1 metre deep Or use a spawning net in your main pond. Don?t feed the parent fish for a few days before you expect the fish to spawn.

Prepare your growing on pond(s) by seeding them with chicken manure or (if you are worried about your koi catching salmonella!) use leaf mould to encourage the production of infusoria and daphnia which the koi fry will need in abundance from day one.


Spawning usually takes place 1 ? 3 days after introducing male(s) to the female(s). It can take place at any time of day or night, unfortunately, typically spawning seems to happen around two am (about the same time as most babies are born!). Spawning can be triggered by temperature changes, change in water conditions, introducing new fish or barometric pressure changes (i.e. thunder.)

Water Conditions

Koi will spawn when the water temperature reaches around 18 Deg +. As always, you need to ensure prime water conditions with zero ammonia or other pollutants present. I have found that the addition of Refresh or similar clay products to the water can to get them going ! To achieve and maintain the necessary water temperatures earlyish in the season (late May) it is a good idea to provide heated conditions so that you can achieve a spawning as early as possible to maximise the length of the growing season.

Spawning media

A good quantity of media is required to collect all the eggs and encourage spawning. We would recommend using purpose made spawning brushes (see picture left).These are soft and easy to handle. To use these brushes in pairs one above the other is often recommended .

Handling Eggs

Remove eggs as soon as they are laid to prevent parents eating them. This is obviously easy to do if you have used spawning brushes. Move eggs on the brushes to a vat or separate tank for hatching. This should contain water of approximately the same temperature as the spawning pond and should be well oxygenated. It does not however need to be filtered. Eggs introduced straight into growing on ponds are vulnerable to predators such as snails, tadpoles and dragon fly larvae etc. Having worked so hard to achieve the spawning in the first place, don't use them as a food source for the local pond wild life!

Handling Fry

Once the fry have hatched, after about 4-5 days, depending on temperature you will need to move them to the growing on ponds. Fry can be moved even when a few days old using plastic bags or fine mesh net. Handle with care however since they are easily damaged.


Now comes the tricky part. To cull fry successfully you must: a. know the parent stocks. b. be Japanese! c. have extremely good eyesight. d. have a modicum of experience. However there are a few basic principles that should be followed. For example, Utsuris and Showas should produce black fry. Whites and reds develop. You should destroy any fry which are not black. With kohaku /Sanke fry will tend to be orange or orange and white. Any pure white or black should be culled. If you are breeding patterned fish you should generally speaking destroy all plain coloured fry. Personally I find that the fry need to be at least 6-8 weeks old before I can see what I am doing! The Japanese breeders start culling generally at 4 weeks. As the fish get larger, cull out any with deformities, missing fins, two heads etc. Ideally you should cull several times in the season, but in practical terms I find that Herons and Kingfishers do more culling than I do!. Culling is clearly important to reduce numbers to ensure that the fish that you want reach a reasonable size by the end of the season..


The fry need an ample supply of live, natural food if they are to survive and grow quickly. Providing the proper growing on pond environment is the surest way to provide a good supply of daphnia and infusoria, on which the fry will develop. Whilst tempting. no dry or artificial food should be given to fry for at least six to eight weeks ? it can damage the gills and is also likely to pollute the growing on pond. Any slight build up of ammonia or nitrite will kill fry very quickly indeed.

Growing On.

Fry normally reach between 2 and 4 inches by mid October assuming spawning in early June. Factors which will affect growth rates are: -temperature, food supply, volume of fish and variety. If you don't cull, you may end up with a lot more babies, but they will be very much smaller than desired. Also of course, your food source will be depleted that much faster.

Unfortunately Runts and plain fish grow quickest !. Showa, Utsuri, Kohaku and Sanke seem to be slowest. Another reason why culling is so important.

The growing on environment is also important since baby koi don?t grow well in man made filtered ponds because of a lack of natural food and are vulnerable to pollutants in water especially nitrite.


An interesting article on early spawning late March Experimental spawning

From the Pond Doctor More on spawning and breeding Koi


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