Koi are large Japanese fish renowned for their colour patterns. Professional koi breeders can fetch hundreds of dollars for a single koi with desirable characteristics. You can learn the methods that the professionals use to breed koi. Once you have a breeding tank set up, you need a spawning mat. You can make your own with the right materials. If everything is done right, you will be a successful breeder from the first batch.
Purchase any type of nontoxic, hardy aquatic plant well in advance of spawning. It should be thick, and about 4 inches deep.
Place the plants in a quarantine tank and treat for parasites. Use commercially available aquatic plant pesticide, which can usually be applied via dropper to your tank's water.
Follow the instructions on the bottle to determine how long you have to quarantine the plant so it isn't dangerous to your fish. Alternatively, you can use tightly packed Spanish Moss or frayed nylon rope to create the mat. Some breeders even use evergreen branches. Generally speaking, anything that is nontoxic and about 4 inches deep will work. The material also needs to be thick enough to catch all the eggs.
Place the mats on the bottom of the tank, covering at least 2/3 of the tank's surface. Koi eggs fall to the bottom of the tank, and will stick to the first thing they encounter. You want the eggs to stick to your spawning mat. If your spawning mat or mop is dark in colour, it will be easier to see your koi eggs and the fry that hatch from them.
Allow the fish to spawn. Generally this will occur in the morning as the water temperature rises. You must be aware of what is going on, because within half an hour the spawning will end and the parents will begin to eat the eggs. As soon as spawning is completed, you can remove the mats and place them into their own tank.
The parent fish can be reintroduced to the babies after they have reached about 3 inches in length. If your fish won't spawn, before changing the mat material, make sure you have males and females and make sure they are all sexually mature (3-4 years old). Feed live food to encourage spawning.
Disturbing the fish during spawning can halt fertilisation and is strongly discouraged.