Like most fish, koi reproduce through spawning in which a female lays a vast number of eggs and one or more males fertilise 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 will nonetheless exhibit a wide range of colour and quality.

Koi will produce thousands of offspring from a single spawning. However, unlike cattle, purebred dogs, or more relevantly, goldfish, the large majority these offspring, even from the best champion-grade koi, will not be acceptable as nishikigoi (they have no interesting colours) 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 (perhaps fed to other fish), while older culls, within their first year between 3″ to 6″ long (also called “Tosai”), are often sold as lower-grade ‘pond-quality’ koi.

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