Like most lovers, kois need privacy to mate.
Koi privacy. A koi breeder only needs two males and a female for optimal spawning. When it’s time for the female to spawn, the male koi will rub, nudge or even bash her belly to help expel the eggs. The privacy is not only to maximize spawning, it’s also to preserve your excellent brood stock for several more breeding seasons.
Spawning is an exhausting, stressful experience for the female. Coping with two males as well as the other kois in the spawning fray will weaken Mama Koi. After laying her eggs, the female will usually be breathing heavily and hanging her head down.
To make sure that kois remain undisturbed, keep a dedicated spawning tank. You could either use a separate tank or put a net inside the pond. Either way, the tank must be maintained at 73 ºF (23ºC) since any change in water temperatures or conditions can deter spawning.
Life after hatching. After spawning, transfer the eggs to an incubation tank. Ideally, it should have about 100 gallons of water and made of non-toxic materials. Treat the waters with malachite green, a chemical antiseptic, to protect the eggs from fungal or bacterial infections.
Eggs hatch at 68-78 ºF (20-21 ºC). Keep the temperature in your tanks constant as extreme temperatures will affect incubation time. Low temperatures will slow down incubation and higher ones can quicken it, leading to deformities in the fry.
The eggs usually develop for 2-4 days. Once hatched, the koi fry will instinctively go hiding. Cover the tank with a net to protect them from predators.
Feeding baby koi. The newly-hatched fry don’t have any swim bladder, mouth or any openings so the water must be well-aerated. They breathe by absorbing oxygen through the blood capillaries in their yolk sac; poor oxygen levels can slaughter your fry en masse.
The yolk sac will be the fry’s primary food source for 2-3 days, or until it has been fed out. They can be given food once the fry start coming to the surface for air or when they’re already swimming freely mid-pond.
About the third day, you can already feed the fry with food. They still don’t have any taste buds so they’ll look for food by sight. The ideal first day food is the yolk of a hard-boiled egg; though it lacks nutritional value, it increases fry size. The fry can also feed on brine shrimp or other planktons. In another week, they should be ready for a mash diet or small pellets.
Culling. Maintain the heat in the incubation tank at 68-78 ºF (20-21 ºC) to encourage growth. Note, though, that kois lose their characteristic colors with too much heat.
Once they are an inch or two in size, you would have to watch out for cannibalism. Migrate the larger ones to another holding tank to prevent the larger fry from feeding on the smaller ones.
In 4-6 weeks, the growing koi should be ready for culling. Culling will eliminate the mono-colored, poorer quality kois and thereby keep the integrity of your brood stock.