If you are interested in breeding koi, you need to have the koi larger than 25cm or else it is biologically impossible because the ovaries and testes in the male and female have not fully developed making them sexually imature. Anywhere after 25cm in length you can start to breed the koi fish. To pair the fish you should have 2 males to 1 female as the ratio. Koi will only start breeding in the mid spring through summer. After you see the koi chase eachother this means they are probably going to mate. A female will then look for a place to put her eggs. This is the time to lower in some nylon ropes which have been frayed at the end. These nylon breeding ropes are a great source for the female to lay her eggs because there is so much surface area for the eggs to attach. Once you see the eggs on the ropes you should remove them into a vat or an incubator before the koi eat their own eggs. Now you have to raise the fry.
Raising the Fry
To raise the fry there are several steps you need to take each day. Before placing the fry into the incubator, make sure the water is treated without chlorine and other toxic heavy metals. Also place 0.2mg/litre of malichite green to kill of any fungus that try to attack or eat the eggs. Also you need to have a faucet placed above the tank so that you can have a trickle of water running in the tank. An aerator would also be a good idea to have as oxygen is needed for the development of fry. Attach the aerator to an airstone. Then attach string 5cm below the water surface going across so that the breeding(nylon) ropes can be held 5cm under the water. The temperature for raising the fry should be at 20-22°C/68-71°F. If the temperature is kept any higher the eggs with hatch faster, but there is more of a chance that there will be a deformation or loss of coloration and pattern. After a day or two of keeping the fry, you will start to see very small eyes of the koi, that is the signal to turn on a small trickle of water and keep that running until the fry have hatched. You may need an overflow if you have the water trickle on. When the eyes of the koi seem very shinny, it means that hatching time is soon. Once the eggs hatch the fry will instinctivly seek shelter such as the ropes. Oxygenating the water is very important at this stage. After two or three days of being hatched the fry will go up to the surface of the water and take a breath which means that they are no longer absorbing oxygen through their yolk sac. They are now able to go swim in mid water. Once all the fry seem to be congregating around the airstone this means they are able to be fed.
Feeding the Fry
The fry get their food by sight not by taste because they have not yet developed their taste buds. This means that you need to have a lot of food on the water surface. Try using live bearers food which is like powder and it will cover the surface of the water easily. Live brineshrimp is also a good type of food the fry. Now since you are feeding the fry, the tank has a chance to get dirty and full of ammonia and nitrites, so make sure the bottom of the tank is cleaned and you are changing some of the water often. Once the koi are a bit bigger they are probably ready to move onto an aquarium for further growth.