Commonly known as "red worms" or "fish insects," these are small crustaceans belonging to the order Cladocera. Rich in nutrients and easily digestible, daphnia are considered an ideal natural animal feed for koi due to their wide variety, broad distribution, large numbers, and high reproductive capacity. Common species include Daphnia magna, Daphnia pulex, Moina, and Daphnia carinata. Daphnia primarily inhabit still waters such as streams, ponds, lakes, and reservoirs. Their populations peak in spring and autumn. Daphnia from small oxygen-poor puddles, sewage ditches, or ponds often appear reddish, while those from lakes, reservoirs, and rivers are usually transparent with a slight greenish or yellowish-gray tint. Koi keepers can collect daphnia at suitable times and locations to meet their koi's nutritional needs. When collected in large quantities, daphnia can be dried and stored as feed for autumn, winter, and early spring.

Commonly known as "jumping fleas" or locally as "green hoppers" or "three-legged insects," these are small crustaceans of the subclass Copepoda. Live cyclops should only be fed to larger koi. They are abundant in some ponds and small lakes and can also be collected in large quantities, dried, and stored for later use.

These small aquatic animals, nutritionally rich and grayish-white in color (sometimes called "gray water" in some regions), are excellent feed for newly hatched koi fry. Rotifers are widely distributed in freshwater bodies such as ponds, lakes, reservoirs, and rivers, where they can be collected or obtained through artificial cultivation.

Commonly known as gill worms, these are aquatic oligochaete annelids. They typically live in clusters in the sludge at the bottom of small puddles, rice fields, ponds, and ditches. Aquatic earthworms usually burrow one end into the mud while extending the other end into the water, retracting quickly when disturbed. They are red or bluish-gray in color and serve as excellent feed for koi. When collecting, the sludge containing the worms should be brought back, repeatedly rinsed with water, and the worms individually picked out, cleaned, and then fed. With proper care, they can survive for over a week.

The larvae of mosquitoes, commonly found in rice fields, ponds, ditches, and puddles, especially abundant in spring and summer. They often gather at the water surface to breathe, diving to the bottom when disturbed and resurfacing after some time. Mosquito larvae are a favored feed for koi. They can be collected with a fine net, requiring swift movements, and should be rinsed with clean water before feeding.

The larvae of non-biting midges (Chironomidae), bright red in color and segmented. Bloodworms inhabit the bottoms of lakes, reservoirs, ponds, and ditches and are a preferred feed for koi.

Many species of earthworms can be used as koi feed, but red worms (Eisenia fetida) are most suitable due to their small size and soft texture, making them easy for koi to consume. Red worms typically inhabit warm, moist environments such as compost piles, cattle sheds, under haystacks, or around paper mill waste. They emerge during rain or when soil humidity exceeds 80%, making them easy to collect. On sunny days, they can be dug up from soil. After collection, place them in a container with some water to allow them to expel soil from their digestive tracts. After a day, rinse them clean, cut into small pieces, and feed to koi.

Soft-bodied and nutrient-rich, maggots can be used as feed for adult and growing koi. Before feeding, rinse thoroughly to minimize contamination of the tank and water. Artificial breeding of maggots requires strict control to prevent environmental pollution.

Typically ground into powder and fed directly or incorporated into pellet feed for koi.

Remove the shells, rinse thoroughly, cook, and then finely chop or mince before feeding to koi. Larger koi have strong digestive systems and benefit well from such feed for growth.

Fresh blood from pigs, cattle, chickens, or ducks can be cooked, dried, and processed into pellet feed for koi. Such feed is highly nutritious. Even better results can be achieved by mixing blood meal with wheat or barley flour to create pellet feed.

Fish and shrimp meat from any species can serve as nutritious and easily digestible feed for koi. However, fish must be cooked and deboned, while shrimp meat should be shredded before feeding. Incorporating some flour into the fish or shrimp meat, steaming, and forming it into pellets is an ideal method.

Hard-boiled chicken or duck egg yolks are nutritious and favored by koi. Mixing egg yolk with flour to create pellet feed yields excellent results. For newly hatched fry, egg yolk is often used as a substitute when protozoa or rotifers are scarce. One egg yolk can feed 200,000 to 250,000 koi fry at once. The method involves wrapping the yolk in fine gauze and gently kneading it at the water surface to disperse the particles evenly. The quantity must be strictly controlled during feeding.