Sexing a Koi fish is impossible until it have become sexually mature, something that will usually happen when the fish has grown 10 inches long or larger. Even sexually mature fish can be a bit tricky to sex, but generally speaking a male Koi will be less plump than a female Koi, since the mature female has egg filled ovaries. The body of the sexually mature male Koi will typically look more slim and streamlined, and the male will often have larger pectoral fins than the female. There are however exceptions to these general rules, and you might end up with a fat male Koi and a scrawny female. When you compare a male Koi with a female Koi, you can often see that the first ray of the pectoral fins is thicker on the male and that the pectoral fin is a bit pointier.
When the Koi breeding season commence, the male Koi will develop tubercles on his head and on the first rays of his pectoral fins. In Koi fish, the breeding tubercles will look like small white elevated spots and they can be hard to recognize for the inexperienced Koi keeper. Sometimes aquarists become alarmed and treat their Koi fish for Ich when they notice these small white spots.
If you keep your Koi in an outdoor pond, they will usually spawn when the water temperature increases in late spring or early summer, provided that they are healthy and large enough to be sexually mature. A water temperature around 20 degrees C / 68 degrees F is ideal to stimulate Koi breeding. If you keep a lot of Koi together, a so called “flock spawning” will occur. A flock spawning will give you an abundance of healthy offspring, but most Koi breeders try to avoid flock breeding since the offspring often display less desirable colorations. During professional Koi breeding, the fish keeper will select desirable parent fishes and place them in their own pond. You will need one female Koi and 2-3 males. If you don’t want to dig a new pond especially for Koi breeding, a small swimming pool intended for children can be sufficient. To increase the chances of spawning you can perform frequen twater changes and low the water somewhat. Feed your Koi live meaty food, such as worms and flies.
Koi is an egg-laying fish and the parents will not hesitate to eat eggs as well as free swimming offspring. During professional Koi breeding where a large survival rate is important, the eggs will therefore be placed in their own pond. Collecting the eggs will be difficult if you let your fish deposit eggs all over the pond, and most breeders therefore use spawning mops. A spawning mop can for instance be made from frayed nylon ropes or synthetic wool. Let the spawning mops float on the surface. The breeding Koi fish will view the spawning mops as floating plants and most likely choose them as breeding site.
If you manage to get your Koi into breeding condition, you will eventually notice that the males are chasing the female. This behavior will usually go on for several days before any eggs are released. When the female deposit her eggs at the spawning mop, the males will immediately fertilize them.
The spawning mops can then be removed and placed in the fry raising pond. During Koi breeding, it can be a good idea to add 0.2 mg of malachite green for each liter of water in the fry raising pond to prevent fungus attacks. You should also equip the fry raising pond with airstones or similar, since Koi eggs need well aerated water to develop. During Koi breeding, some Koi keepers use mechanical filters in their Koi raising pond, but a problem with this is that small Koi fry might be sucked into the filter. You can prevent this by placing a fine net around the water intake. Foam filters is a good idea in a Koi fry raising pond when the eggs have hatched, since microscopic food particles can grow on the foam. It is also possible to refrain from mechanical filtration and instead perform very frequent water changes. Replace roughly 20 percent of the water during each change.
Koi offspring will usually hatch after 3 to 7 days, depending on the water temperature. If the water temperature is around 20 degrees C/ 68 degrees F, the eggs will hatch after 3-4 days. You know that the hatching will take place within a day when the eggs develop a special shiny look. Eventually, you will be able to see how the fry wiggle and struggle within the eggs, and after a few hours they will escape.
As soon as the Koi fry have emerged from the eggs, they will attach themselves to the sides of the pond. The Koi fry is equipped with a sticky pad that makes it possible for its head to stay attached to a surface for 2-3 days. As mentioned above, the eggs need plenty of oxygen, and this is true for the Koi fry as well. Proper aeration is very important for successful Koi breeding. After 2-3 days of attachment, the Koi fry will swim up to the surface to get some air. The air is pressed into the swimbladderof the fry, and after this the Koi fry will be able to swim around in the pond. Until this happens, you do not have to feed your Koi fry.
Very small Koi fry can be fed infusoria, newly hatched brine shrimp and daphnia. Live food is recommended for Koi breeding, but hard boiled egg yolk is also a good choice during the first few days. If you can not provide your fry with this, you can instead use prepared food. Only use special Koi food with high protein content and turn it into a fine powder before you feed it to your fry. Ideally, you should also add some spirulina and pure wheat germ to the diet. After roughly 7 days, you can start feeding your fry special fry food, e.g. fine powdered pellets. When your Koi fry have reached a size of 0.5 inch you can train them onto small pellets and gradually increase the size of the pellets until the offspring eat the same food as their parents.
It is easy to over feed Koi fry and cause the water quality to plummet. The servings must therefore be limited and you should perform a water change every day. The easiest way is of course to use water from the pond where our keep you adult fish, as long as you know that they are healthy and that the water quality is high. Young Koi is very sensitive to ammonia, and you should keep a watchful eye on the water quality to prevent fatalities during Koi breeding.
