Editing Koi (section)

== Breeding ==
{{Unreferenced section|date=January 2024}}[[File:Koi feeding, National Arboretum.jpg|thumb|right|Feeding of a large school]]
When koi naturally breed on their own they tend to spawn in the spring and summer seasons. The male will start following the female, swimming right behind her and nudging her. After the female koi releases her eggs they sink to the bottom of the pond and stay there. A sticky outer shell around the egg helps keep it in place so it does not float around. Although the female can produce many spawns, many of the fry do not survive due to being eaten by others.

Like most fish, koi reproduce through spawning in which a female lays a vast number of eggs and one or more males fertilize them. Nurturing the resulting offspring (referred to as "fry") is a tricky and tedious job, usually done only by professionals. Although a koi breeder may carefully select the parents they wish based on their desired characteristics, the resulting fry nonetheless exhibit a wide range of color and quality.

Koi produce thousands of offspring from a single spawning. However, unlike cattle, purebred dogs, or more relevantly, goldfish, the large majority of these offspring, even from the best champion-grade koi, are not acceptable as ''nishikigoi'' (they have no interesting colors) or may even be genetically defective. These unacceptable offspring are culled at various stages of development based on the breeder's expert eye and closely guarded trade techniques. Culled fry are usually destroyed or used as feeder fish (mostly used for feeding [[Asian arowana|arowana]] due to the belief that it will enhance its color), while older culls, within their first year between 3 and 6 inches long (also called ''tosai''<ref>{{Cite web |last=Doctor |first=Atsushi Ueno |date=5 December 2023 |title=Research On Tosai Koi Fish |url=https://zenkoifarm.vn/ca-koi-tosai-la-gi/ |access-date=9 February 2024 |website=Zen Koi Garden research center |language=vi |archive-date=16 May 2024 |archive-url=https://web.archive.org/web/20240516033050/https://zenkoifarm.vn/ca-koi-tosai-la-gi/ |url-status=live }}</ref>), are often sold as lower-grade, pond-quality koi.

The semi-randomized result of the koi's reproductive process has both advantages and disadvantages for the breeder. While it requires diligent oversight to narrow down the favorable result that the breeder wants, it also makes possible the development of new varieties of koi within relatively few generations.