With many types of aquaponic fish available, you must put some thought into the purpose of the fish you choose. In aquaponics, some fish are raised to be eaten, and some are simply just to create a symbiotic relationship with the plants. Koi are not considered a good fish for eating, so for those looking to harvest not only plants, but fish as well from your aquaponics system, you might prefer a different fish such as Tilapia, Channel Catfish, Bluegill or a crustacean such as the Redclaw Crayfish or Prawns. Those who prefer not to harvest their fish may want to consider Koi.
The Japanese word "koi" means "carp." It includes both the dull grey fish and the more well-known brightly colored varieties. The fish that are best known as Koi in English are referred to more specifically as Nishikigoi in Japan, which means "brocaded carp." In Japanese, the word "koi" is a homophone for another word that means "affection" or "love." Therefore, Koi have become symbols of love and friendship in Japan.
Koi are one of the most popular fish used in aquaponics due to their long lifespan, their ability to easily live and breed in your system, their strong resistance to diseases and parasites, and of course, their attractive colors. Koi are omnivorous fish which means they will eat just about any food you give them – including algae, debris and plant matter that falls into their pond or tank, so in many cases, additional feeding is not necessary. You can also feed them peas, lettuce and watermelon.
Koi are a colored form of Amur Carp – also known as Cyprinus Rubrofuscus – that are kept for decorative purposes in outdoor ponds or water gardens. They are a cold-water fish, and their ability to survive and adapt to many climates and water conditions has allowed them to work well in aquaponics systems. While Koi are cold-water fish, they benefit from being kept in the 59° to 77 °F range, and they do not react well to long, cold winter temperatures. In general, Koi can function in temperatures ranging from 35° to 85° F.
While a Koi fish can grow to as long as 3.28 feet, average lengths of up to 2 feet are more often achieved after four to five years if water quality and space in the tank are optimized. Koi also enjoy long lives, with lifespans ranging from 25 to 35 years. The common carp is a hardy fish, and Koi retain that durability.
Feeding is not recommended when the water temperature drops below 50 °F. Care should be taken by hobbyists that proper oxygenation, pH stabilization, and off-gassing occur over the winter in small ponds, so they do not perish. Their appetites do not come back until the water becomes warm in the spring.
Reproduction Like most fish, Koi reproduce through spawning during 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. Thousands of offspring can result from a single spawning. When Koi naturally breed on their own, they tend to spawn in the spring and summer seasons. The male will follow the female, swimming 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 eggs helps keep them in place so they do not float around. Although the female can spawn many times, many of the fry do not survive because they are eaten by others. If the egg survives for four to seven days, the fry will be hatched from the egg.