Guide to Koi Rearing and Breeding

The common carp is the forerunner of our present day Koi. Koi are not big goldfish. The goldfish is a distant cousin to Koi. The Japanese name for Koi is Nishikigoi. Nishikigoi were developed by the Japanese over 200 years ago. These gorgeous creatures usually live in lushly landscaped fish ponds outdoors. The ponds are constructed to provide adequate oxygenation and filtration of the water. These ponds make an excellent landscaping addition to a garden.

The average Koi can grow to 24 – 36 inches! The size of the pond, the amount of aeration, and feeding methods will affect the growth of the fish. It is not uncommon for a small Koi to grow 2 – 4 inches a year in a backyard pond.

Koi Classification

  • Kohaku – A koi with a WHITE body and RED accent pattern on the back. The Kohaku is the most popular variety of koi.
  • Tashio Sanke – A tri-colored koi with a WHITE body with a RED pattern and BLACK accents on the back.
  • Showa Sanke – A tri-colored koi with a BLACK body with RED and WHITE accents on the back. The modern day Showa has more WHITE on its body like the Tashio Sanke.
  • Bekko and Utsuri – A non-metallic koi with Black on White, Red, or Yellow body.
  • Asagi and Shusui – The Asagi is a non-metallic koi with a BLUISH reticulated, netlike scale on top, with a RED belly. The Shusui is the same as the Asagi except it has German (or Doitsu) scales.
  • Tancho – A koi with a red spot on the top of its head, and no red on its body.

Koi Ponds

A Koi pond is an enclosed, recirculating, freshwater system for keeping Koi (Japanese fancy carp). The the purpose of a pond is to provide a healthy home for Koi. Koi can live in the others, but not in the numbers that necessitate a Koi pond.

A Koi pond should serve two main functions;

  • One is to provide a healthy home for your Koi.
  • The other is to provide “clear-water” so that you, the owner, can enjoy your Koi.

Neither of the above is dependent on the other. You can have healthy and happy Koi in a “pea-soup” green pond, or you can have diseased and dying Koi in green water. Having “crystal-clear” water is not an indication of a healthy pond. Water is an excellent solvent. It is possible to have unhealthy, toxic, crystal-clear water. In fact, you might have pond water so unhealthy that even algae would not grow in it! In addition, most Koi ponds are an attractive addition to the garden!

Design Considerations for a Koi Pond.

  1. 500 gallons or larger. “The bigger, the better.”
  2. 24 inches or deeper. Deeper is better. Does not take up any more space or proportionally more filtration.
  3. Straight or near vertical walls. Protection from predators and more pond volume.
  4. One or more bottom drains.
  5. A bottom that slopes towards the drains and away from water falls or incoming water.
  6. Some form of surface skimmer. The pool and spa skimmers with a “weir” work best.
  7. A biological filtration system.

Introduction to Filtration

Most ponds have been built as part of the general landscaping. Later on, some goldfish were added to give the pond some life. Then those beautiful Koi were discovered. But now suddenly, when there is brightly colored fish to look at, green water is a major problem.

What is needed is a biological filter. This will take the wastes from the fish and process them naturally to produce clean water. This is done through micro organisms that develop in the biological filter.

As stated above, a swimming pool filter will not work on a fish pond. The reason is, in a swimming pool you use chlorine, and the pool water has very little solids and ammonia (fish wastes) in it. A swimming pool filter used on a fish pond will require frequent back-flushing. Also a high powered pump will be required to push the water through the filter. Remember, you will need to run the pump 24 hours a day, and this will show up on your electric bill.

Koi Diseases

A Koi’s health depends upon the environment provided by the human owner. Koi have a high resistance normally and succumb to disease usually only after exposure to stressful conditions that break down the normal immune system. A stressed fish becomes a sick fish. It has been said that:

Fish Disease = Stress Condition + Disease Agent

Stress is the main factor man has the most control over. Many disease causing organisms normally occur in the same environment as the fish. They usually only become a problem when present in significant quantities and/or stress occurs. Therefore, by controlling stress you can help maintain a healthy pond. Prevention is easier than treating your pond for disease.

Koi Breeding

Many koi keepers do not wish their koi to breed, as the spawning event is quite stressful for koi. Unlike at all other times, koi are not graceful or reserved in their spawning behaviour. If males significantly outnumber ripe females then females in particular can become quite exhausted and physically damaged as the males drive and bash at the swollen females in their attempts to expel their eggs.


Koi need not spawn in order to remain healthy. On the contrary, the process has accompanying dangers to health, color, and patterning of fish which are involved. Under normal pond conditions, indiscriminate and spontaneous spawning may occur, unobserved by the pond owner, and the resulting eggs either are eaten by the adults or disappear down the overflow, particularly when the pond is free of vegetation. When spawning is a deliberately encouraged and planned event, many more considerations are involved.

Age as a FactorA procedure which works for one person usually can be found to have an alternative used by another successful breeder. However, one consideration must be recognized by all breeders: age of the fish. A young koi may be fertile, but the hatch which it produces of young fish is not strong. On the contrary, although a female may have a span of fertility over 15 years, as she ages, her eggs develop a tougher covering which sperm have difficulty in penetrating, so the hatch may be unpredictable and quixotic.


The number of males to female in mating is controversial. Certainly, a 1:1 relationship has a more predictable genetic outcome than when several males are used. Equally important as a consideration is the possibility that a number of aggressive males may subject the female to injury in their spawning excitement. Under NO circumstance should the ratio ever exceed 2:1. Taken for granted is that fish being mated will be of relatively similar size.


Mated koi are separated from others into their own tank or pond. Isolating specimens actually is helpful if done about a month ahead of the of the possible spawning. A location should be chosen where the fish readily can be picked up and where they can be calm. Hand-feeding helps assure them as well.

Setting the Stage

Female koi become large and bloated with eggs and usually are readily identified. Their abdomen is heavy and soft but resilient. When they are ready to lay the eggs, the male is presumed also to be ready. Because the heaviness of the female partially is buoyed up by the water support, she never should be out of water at any time when netted or moved by hand. Male or female fish being mated must be handled with utmost carefulness.

Some breeders put both male and female into their separate pond at the same time. Others put the female in early, to get her acclimated, then add the male later in the afternoon or early evening.


When the female is interested in spawning, she makes “household” moves with the plant/plastic medium, as though building a nest. The male is attracted to the “spawning position,” in a type of courting or foreplay. If there are two males, they sandwich the female between them. A single male tries to force her against the side of the pond. The males thrash and bump the female, literally forcing out thousands of her eggs while simultaneously excreting sperm. After the first discharge of eggs, if there is but one male, it positions itself on the opposite side and helps the female get rid of the remaining eggs by a second session.

The turbulence of the action scatters eggs all over the pond or pool. They are very sticky and adhere to anything they touch: nesting material, pond walls, bottom. Left alone, thousands upon thousands will eventually hatch.

As a result of the frenzied and violent action, the female may be injured, occasionally getting a split fin or tail. She may also retain some of the egg residue, which can be very dangerous. Additionally, the insensitive male may continue driving her, adding to the problem of her beaten condition. Therefore, the breeder should observe the fish at 15 minute intervals during the 4-6 1/2 hours consumed by the process.


Upon the completion of spawning male and female seem interested only in eating the eggs. If the male continues to be aggressive, he should be removed, carefully. He now is harder to handle and is vulnerable because his usual protective, slime-layer body covering has been worn off, so that his normal slippery feeling gives way to a sandpaper-like touch. He also probably will fight any netting. At this time, he can be prey to fungus or disease, and best can be protected by returning him to high quality, regular pond water.

The female should be isolated in a pond by herself, with other females, or with young koi for at least 24 hours, so she can recuperate. She will lose the spawning odor and have a chance to rest in this length of time. Otherwise, she may be subject to further harassment by males not previously involved. Watch jumping; for some reason, recently-spawned females have a tendency to jump, even when alone.


diagram 2 Japanese breeders discard about half of the hatch on the premise that there are just too many to care for. This decision is a matter of individual choice, in anticipation that about 50,000 fish are in the usual hatch.

By the third day, fertile eggs are clear, while the infertile remain cloudy/chalky/opaque and eventually develop a fine, hairy growth. Two black dots, the future eyes, are visible. In 4-5 days, a multitude of jerking “commas” report live individuals. Unless there are very cold nights, all fertile eggs will have been hatched by the sixth day. Now is the time when they are vulnerable to the dragon-fly larvae that hatch.


Feeding now presents a problem, since it must be accomplished without filtration and without polluting the pond. For the first 24 hours, the egg sac supplies sufficient food. Following that period, for the first three months, a feeding is required five times daily!! Hopefully, green algae will develop to provide supplements of natural food.



  1. By ricardo murillo jr


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