How to raise home-bred koi

If you are a keen gardener like me, you love to try new ideas that will add further interest to your garden, encouraging transient wildlife to stay a while or even take up residence. Trees can be planted, bird boxes and feeders erected all in the quest to add another dimension to the lives of birds and other wildlife and those of ourselves. What better reward could there be than to think ‘My efforts may have lead to new life taking up residence and perhaps rear their young in my garden’.

There are parallels in the koi pond. Ponds can offer many rewards to their keeper, irrespective of what their ambitions for size, design and planting of the pond may be. In a similar fashion by which birds can be encouraged to take up residence, nest and produce young in our gardens, fish too can be encouraged to breed and produce additional offspring for our ponds. A further benefit is that unlike birds, any homebred  aquatic additions to the pond are ours to keep and enjoy, being safely retained within the boundaries of a pond.

Breeding koi in our pond, besides being a free source of fish can also enrich the pond keeper’s life. They offer the fulfilment of bringing new life into a pond, the sheer intrigue gained by watching the development of these homebred koi endeavouring to work out what variety they are likely to be, and which fish in the pond are likely to be their parents.

What does it take to assume the role of a surrogate mother (or father), successfully breeding and growing on homebred koi in a koi pond? The process is a logical one and if the pond and fish are prepared sufficiently well, then success can be achieved by assuming quite a passive role, letting nature take its course with these ornamental beauties.


Koi and other pond fish are very closely related and despite their apparent differences in shape, size and colour are quite compatible with each other. All goldfish varieties, ranging from the common goldfish through other colour variations of the blue shubunkins and the fancier-finned varieties will interbreed, as they have all been selected from the same species. Different cats and dogs will interbreed to produce moggies and mongrels, which just like fish, will test even the acutest of powers of deduction when trying to identify their parents. Homebred koi can prove to be a real Heinz variety!

Koi will breed in ponds during late spring and early summer, in response to lengthening days and warming water. Mature females will begin to swell up, becoming plump with eggs, while males remain trim, more agile and alert, tending to dart through the water with greater energy.

As the eggs within the females begin to ripen, ready for spawning, the females release pheromones into the water that the amorous males (which rarely require much encouragement) will find irresistible. The males will swarm around any ripe female (an even some non-ripe females), nudging, bumping and bashing any fish that is suspected as being a female ready to spawn. A pond should be prepared to cope with this physical event by being heavily planted with submerged aquatic weed, such as Elodea and Hornwort or artificial spawning media such as a spawning mop. Try to remove as may sharp or abrasive corners or edges and place any planted baskets where they may not cause females to come to harm. Spawning behaviour can last over several days, with the more intensive chasing activity occurring for an hour or so each day. During these bouts of spawning activity, koi appear oblivious to the dangers of being injured on sharp corners or edges, and can often be left with damaged flanks and fins.

All female koi (being members of the carp family) will release thousands of tiny translucent adhesive eggs as they drive in and out of dense vegetation. It may be necessary to remove females once they have spawned as the persistent attentions of eager males will only further exhaust and stress ‘spent’ females.


Once the eggs have become attached to the various submerged surfaces (including some on the spawning mop), assuming they have been fertilised by the males (of whatever variety) they will begin to develop, hatching into larvae after 4-5 days (depending on the temperature). Even at this stage, it is quite common to be oblivious to the presence of thousands of tapioca-like eggs nestling in the weed or attached to the pond sides. For this reason, always try to provide an area of dense planting, particularly in the shallower areas, as this will provide good spawning area whether you are ready or not.

Koi release many thousands of eggs because of the low odds of producing a mature carp from a single spawn. Fortunately, this means that if we offer even a little extra support for the eggs and fry over and above what they would ‘naturally’ be afforded, we are likely to be rewarded with a good number of fry – and who knows, even a recognisable variety!

Perhaps one of the most surprising hazards for fry are other fish in the pond (including their parents) which, despite their scavenging lifestyle, will sooner eat one of their offspring than any other aquatic live food. Dense planting serves a further purpose after being used as a spawning media by providing a refuge or ‘nursery area’ for these tender little fry. Submerged weed will provide fry with a safe bolt-hole as well as a labyrinth of surfaces on which to graze and gain essential nutrients. Alternatively, to increase their rate of survival even further, you could move the egg-laden spawning media over to a purpose-built and prepared mud pond (see Boxout).

Several weeks after hatching, the fry become bolder as they increase in size and courage, venturing more frequently out of the safety of the vegetation. It is common for new fry to be seen for the first time at this stage. They are still likely to be displaying distinctively non-ornamental livery, blending in with the subtle colours of their surroundings and because they make good use of nature’s camouflage, it can be common to become overrun with 1” naturally-coloured fish. If this is the case, then a decision should be made which will improve the growth rate of all the new-born fry. The more fry the slower their growth rate, and if your new members of the family are going to become permanent fixtures, they need to attain as large a size as possible prior to the autumn to stand a better chance of over-wintering safely. Koi fry growth can be enhanced through thinning their numbers and passing on those that are surplus to other pond owners.

The race against time.

As most fry, besides being a surprise occurrence in a pond, will be produced in June or July in a garden pond, they will only have 10-12 weeks to reach a size at which they will easily overwinter. The larger a fish can be by the time it stops feeding in the autumn, the greater its chances of overwintering.

Fry growth is also affected by feeding a suitable diet. If you’ve created a greenwater-filled nursery mud pond for them, then they will feed on highly nutritious live food and grow accordingly. If reared in your main pond, on account of their tiny mouths and simple digestive tracts, a soft and easily consumed diet such a crumbled flake food is an ideal food. This soon softens up and sinks to tempt even the most reluctant of feeders.

Ponds that are at least 3’ deep will overwinter fish more successfully than shallower ones as they offer more stable water temperatures. The chances of safe passage of your fry through even the harshest of winters can be improved by adding a small pond heater to prevent ice from completely covering the pond.

After winter.

Assuming that you have managed to bring your fish, both old and new, safely through the winter, it is likely that some real changes in the appearance of your fry will start to be observed.

During this period, as the fish start to feed again, it is likely that some of the previous year’s fry will start to show patterns and depth of colour, with oranges developing into deeper reds. Such are the quirks of koi genetics that some pond-bred additions may never actually develop the typical ornamental colours of their parents, with many reverting back to an orfe-like mono-coloured form. It will not be for perhaps another 18 months to 2 years until your homebred fish are mature enough to start their second UK generation, and by that time you should be proud if you’ve been able to grow them to 6-8”.

Boxout 1.

Desirable features in a koi pond that will improve the chances of rearing homebred pond fish.

1. Slack water areas. Provide back water areas where fry will naturally congregate and take shelter, away from the every day hustle and bustle of pond life.

2. Densely planted shelving and shallow areas. Planted nooks and crannies provide vulnerable fry shelter and areas for trouble-free feeding.

3. Protection from the pond pump. Several models of pond pump are designed to handle solids of up to 6mm. This will include fry. Try to protect fry from pumps by fitting a pre-filter to any such pump so that it will not allow fry to pass through to the pump.

Boxout 2

How to make a fry pond

The simplest way of improving the chances of survival of fry from a pond spawning is to provide a natural fry pond for them.

Ideally a fry pond should be dug out of clay as this provides an excellent environment for culturing fry food naturally. If clay is not available then at second best a liner can be used. Try to provide as large a surface area as possible with a depth of between 2 and 3 feet.

In May, fill the pond up with water, aerate and add a few handfuls of well-rotted manure or topsoil and a sprinkling of inorganic fertiliser. The pond water should turn green in 1-2 weeks and will be ready for fry in 4 weeks.

If the pond is filled too soon then predatory invertebrates such as damsel fly larvae will have colonised leading to poor fry survival rates. If it is filled too late then sufficient live food to support the fry would not have been produced.

After spawning has taken place naturally (June/July), place eggs in the pond and do not add any dry food for at least the first week.

After their first few weeks in the nursery ponds, the fry’s diet should also be supplemented with a powdered dry food. This is because the fry will have consumed most of the live food that the pond initially provided and it is a race to try and progress the growing fry onto a small floating pellet as soon as possible.

Their dry supplementary diet is a high protein koi food ground down to different particle sizes using an industrial coffee grinder. The small fry start off with ‘espresso’ and move onto a coarse ‘cafetiere’ grind until they take a mini floating pellet after which their growth rate will rocket again.

Kill blanketweed and string algae.