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SELECTING THE PARENTS FOR BREEDING
For the novice breeder the question of selection can be frightening and sometimes bewildering. You can, although it's not advisable, to breed koi when they are too young.
The best results come when the female is at least 4 years old and a male at least 3 years old.
The reason being is that the female would be of a good size and in good condition which would let her carry many thousand large, quality eggs with a good yolksac. The larger the yolksac the larger the fry and the quicker they will eat microscopic food so the faster they will grow.
It is an advantage to use a large koi of 4 years as they have better ability to cope with the sometimes many hours of the spawning cycle. As a novice some people get quite upset over the rough handling of the female by the male or males when they are in the breeding tank.
It is quite common for the male to completly lift the female out of the water and push her around the pond sideways. After she is just about empty and has deposited her eggs on the breeding material, she will hang her head and stay in a corner exhausted and some will think she is dead. Still the males will chase her for more action, then when she has no more interest in laying all of the koi in the pond will start eating or sucking at the eggs.
But I am getting ahead of myself lets go back to the selection of parents.

We will leave the exotic koi alone. For the novice, all he or she wants to do is to put down a spawn of their own, so we will only talk about two lines of koi. They are: the ones with white skin - line 1, and the ones with black skin - line 2. Then we will throw in their metallic cousins!

White skin koi usually have white pectoral fins but some may have a few black stripes, as with the sanke, and either – red, orange and/or black patterns.
Some white skin line examples :- kohaku, sanke, tancho, (shiro, ki, aka) bekko.
Black skin koi usually have black in the joint of the pectoral fins and either - white, red, orange or a mixure of all in the pattern.
Some black skin line examples :- showa, (shiro, ki, hi) utusri.
Looking at the photos below, left is a female showing a fat belly (eggs) and the right is a male, slim like a torpedo.
We can use a male metallic koi to breed with both lines if you don't have a mate of the same line. This will make the offspring look like their parents but with a metallic sheen, this will put the offspring into the Hikari class (which means metallic). Some of these hikari are beautiful to look at in the sun.
The above applies to both full scale and doitsu types.
It is advisable to keep a log or record of your mating as you will need it later on. Some things to consider in your log - if the male does his job (stimulates the female), if the female is a good layer (utilises breeding material), and if they were a good match. It is especially important later when you are culling, to keep a record of how successful the mating was (colour, pattern, hatch rate etc.), then you will have a reference of parents and offspring for future spawns. Oh and don't forget to log who the parents were (I give each of my koi an ID number)!
Do not breed koi with deformities, only breed with healthy koi.

The text was written by H. Watson for the A.K.A. Website.

All the text on breeding, spawning materials, spawning ponds, culling and preparation, etc are based on more than 20 years personal experience of success and failures and do not override the advice given by some koi clubs.