Koi Breeding 101

Updated on July 2, 2019
Jana Louise Smit profile image

Jana worked in animal welfare with abused and unwanted pets. She loves sharing her hands-on experience regarding domestic and wild critters.


What You'll Learn About Koi

  • The ups and downs of breeding koi
  • How to prepare for tank breeding (the more difficult option)
  • How to groom the lucky couple
  • What the mating process entails
  • How to care for koi eggs
  • How to breed koi in a pond (the easier route)

The Downside of a Koi Breeding Program

The perks of multiplying your pond include leveling up in the game of koi keeping and maintenance and let's face it, nothing beats the moment you watch the fry hatch. The second best is watching them grow and prosper. All the excitement can push the negative side of a breeding program into the background. Have a look at the “side effects” and decide if they're worth it:

  • The process takes a lot of time, space and money.
  • A single mating produces enough babies to pack an Olympic stadium. You'd have to face the reality of culling.
  • You won't get rich breeding koi.
  • The female is often left with a few injuries.
  • It's exceptionally hard to breed for certain colours and patterns. If that's the plan, you could be setting yourself up for a huge disappointment.

The Equipment

To breed koi outside of the pond, you'll need several tanks (polyethylene is acceptable) and other items.

  • At least two tanks; one for breeding and an additional hospital tank. Both must be able to contain at least 200 gallons of water.
  • Each tank needs its own: filter, water pump, heater, air pump, airstones and a net cover.
  • You will need spawning brushes for the breeding tank.


Once you have the equipment, it's important to find the right place. The project might last weeks and once in full swing, and it might be difficult to move should the location prove undesirable. Make sure you have easy access to the site and easy maneuverability around the tanks. In addition, the place needs to be near a water point and not in direct sunlight.

When Is Breeding Season?

Koi multiply when the weather (or the tank) warms. It's hard to pinpoint any real “season” since optimal conditions can vary from country to country, state to state and even among the fish themselves. In most places, outdoor breeding begins when water temperature warms to around 68 degrees Fahrenheit. Find out your local breeding times. If you know of any koi keepers in your area, they would most likely already have this information.

The Bride and Groom

Females are more robust and plump (foreground) and males tend to have slim, more flattened looks (top of the image).
Females are more robust and plump (foreground) and males tend to have slim, more flattened looks (top of the image). | Source

Preparing the Pair for Breeding

  1. Choose healthy fish; the male should be older than 3, and the best female age window is between 5–8 years (males have no age limit).
  2. Keep the water's surface a foot below the breeding tank's top. Add the equipment, spawning brushes and the female.
  3. Expose her to an environment that mimics the lengthening days of spring. This can be done via natural lighting, if the room has it or the tanks are outside. Optionally, you can use artificial lights with timers.
  4. When the female is ready, she'll investigate nesting areas (like the spawning mats). At this point, you can add the male to the breeding tank and cover the tank with netting.

What Happens During Breeding

Koi romance takes a few hours, and experienced breeders place the male with the female in the late afternoon. Normally, spawning happens just before sunrise. Stay a while to make sure he's not too aggressive. Koi mating can be boisterous, but some boys cross the line and need to be removed for the female's safety.

  1. When she's ready, the female releases a pheromone to excite the male.
  2. He maneuvers her towards the spawning areas.
  3. The female releases her first batch of eggs and the male spews sperm.
  4. The parents then eat some eggs; this natural culling is very nourishing.
  5. The whole process repeats until the female has no more eggs to release. Remove the male at this point, or he might snack a little too much.

Seeing the Difference in Eggs

Unfertilized eggs are easy to spot. They turn white very quickly. The eggs containing the babies are clear enough to see the creatures' eyes.
Unfertilized eggs are easy to spot. They turn white very quickly. The eggs containing the babies are clear enough to see the creatures' eyes. | Source

How to Care for Koi Eggs

The male returns to the pond. The female recuperates a few days in the hospital tank to rest and receive treatment for any physical damage. After spawning, neither gender cares for the eggs or the young. Once the adults are removed, the breeding tank turns into your nursery tank.

  • Water chemistry and temperature must stay optimal
  • Give special attention to ammonia levels. A reading of zero is ideal, and anything higher calls for a water change is in order. Replace 20 to 30 percent over an hour or instead, use an ammonia reducing product
  • A water change is a good idea since the mating process leaves a smell and sometimes clouds the water
  • Don't expose the eggs to air when water levels drop during a change
  • Turn off the water filter and place airstones evenly throughout the tank (make sure they stay active)
  • Remove unfertilized eggs, which will be white and often fuzzy (fertilized eggs are clear with dots, which are the fry's eyes)
  • Keep water conditions stable and don't worry about a slight algae bloom; this provides food for newborn koi
  • The eggs hatch between 3 to 6 days after fertilization.

Breeding Koi in a Pond

This easier option requires no additional tanks. When weather and water conditions are right, koi will mate. Keep in mind, however, that not a lot fry survive this method. Adult koi hoover up any eggs and newborns they can find. This might not be a drawback if you only want a couple of new koi and cannot face culling yourself. Supply nesting sites for the adults and hiding places for the babies. Use things like spawning brushes and ornamental plants that curtain over the edge of the pond. You can also use plastic plants as a substitute for the real thing.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2018 Jana Louise Smit


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