Like most fish, Koi lay eggs instead of bearing live offspring. In fact, Koi are known to lay thousands of eggs at a time. This process may sound easy and promising if you’re considering breeding Koi for fun or profit, but many factors must be exactly right for even a small percentage of these eggs to survive so don’t get your hopes up just yet.   

The first component of Koi breeding is a pair of healthy, mature Koi. If you can raise healthy Koi, however, they will reach sexual maturity as early as three years of age. Sometimes they will not start breeding until they are almost four. It depends more upon their physical growth rate than age. A pregnant female will be relatively easy to notice among other Koi. They will, of course, appear larger, mostly in the abdomen. Pregnancy, however, does not equate to fertility in the fish world. The eggs must be fertilized by a male, and determining when a male is ready to breed is not as easy.  

The next most-important factor is an appropriate breeding setup. Generally, it is preferable to conduct Koi spawning in controlled environment, separate from the other Koi that you may have. Since the water parameters most appropriate to encourage breeding are somewhat stressful on the Koi, it is best not to place the entire flock under this type of strain unnecessarily. Also, when keeping the spawning pair separate from the rest of the flock, you can better ascertain the type of Koi you will produce if you are trying to cross-breed. Furthermore, you will have much more control over the fate of the eggs, which will improve the overall survivability of your Koi babies.  

The ideal setup will have plenty of room for both fish. Expert opinions vary, but temperature should be somewhere around 2-5°C higher. Always remember to give the female an area to lay the eggs; they will need something to attach to. Plant matter tends to work best, but even experienced breeders have been known to use a solution as simple as a mop head to save costs.   

Always keep the eggs separate from the rest of the flock. If left open to the adult Koi, they will likely be eaten. Additionally, it is best if there are some “private” areas within the breeding enclosure, basically anything that gives the impression of a nest. Again, some plant matter or even fish netting will work well for this effect. 

Koi will naturally breed in the spring. They can sense the time of year through changes in the water temperature. It would be considered extremely difficult to simulate this, so it is best to allow this to occur naturally. If the pair will breed, they will very likely do it in the morning. All of the conditions must be just right. If they do not breed, try adjusting the water temperature; it may be too cold or too warm. Make sure they are well fed. You can keep trying, but it is only recommended to keep the Koi in the breeding enclosure for a few days at a time. They may perish if exposed to the high temperatures for too long. 

The eggs, if fertilized, will hatch in seven to ten days after the spawning. While it was said that Koi will lay thousands of eggs at a time, do not raise your expectations that each one will survive. Not all of the eggs are going to make it and you should not expect thousands of little Koi swimming around for your money making. Make sure that you set your standards lower and don’t base your budget on the hopes of thousands of Koi offspring in each batch of two mature Koi.