1. Goldfish: Goldfish and koi fish belong to CYPRINIDAE. They are similar in size and gentle in temperament, so they can be in the same fish tank. But if you want to mix different kinds of goldfish, you need to observe them and avoid attacking each other.

2. Grass carp: Grass carp and koi fish can also be mixed, but not in the same fish tank. Grass carp needs more water space, and an area that can be separated in the aquarium is needed to ensure that koi fish and grass carp will not conflict.

Matters needing attention in polyculture in koi fish:

1. Keep the water clean: Keep the water clean when different kinds of fish are mixed, because different kinds of fish have different metabolites and produce different wastes. Appropriate measures need to be taken, such as using filters and changing water, to maintain water quality.

2. Grasp the number of fish: When polyculture in koi fish, it is necessary to grasp the number of fish and properly control the number of each kind of fish, so as to avoid overcrowding in the aquarium and affect the growth and development of fish.

3. Avoid over-feeding: When polyculture, fish food should be properly fed to avoid fish health problems caused by over-feeding. The normal growth of fish needs more exercise. Overweight fish can cause health problems.

4. Avoid conflicts between different fish: Different kinds of fish are different in nature and food. In order to avoid the conflict between different fish, we should choose fish with similar size and nature for polyculture, and at the same time, we need to observe the behavior of fish and adjust the species and quantity of polyculture in time.

5. Suitable environmental conditions: When polyculture, attention should be paid to the suitability of environmental conditions such as water quality, temperature and PH value to maintain the health and growth of fish. At the same time, it is necessary to change the water quality regularly and clean the aquarium to ensure the clean living environment of fish.