① There is little difference in the requirements of fish for water quality, including water temperature, pH, old and new, hardness and salinity.

(2) The size difference should not be too big, and attention should be paid to choosing fish species that are easy to be mixed, and they should not pose a threat to each other.

(3) Some large carnivorous fish, such as piranha, are better kept alone or mixed with fish with similar body shape and habits, as shown in the figure.

4 Some rare and timid fish, such as colorful immortals, are best kept separately.

⑤ In polyculture, attention should be paid to the coordination of body color and body shape.

⑥ Different fish have different activity spaces, so we should pay attention to the collocation of each layer of fish.

⑦ There should not be too many mixed fish species, otherwise the aquarium will be very messy. It's best to take one kind of fish as the main body, and then decorate a small number of other fish that won't upstage.

Common sense of stocking density: 1 cm fish should be matched 1 liter of water. This is just a general statement. However, different kinds and sizes of fish have different requirements for oxygen capacity. Therefore, it is difficult to generalize stocking density. If the aquatic plants are abundant and the water quality is clear, the stocking density with aerobic facilities can be higher, otherwise it will be less. The general principle is that less should not be more, and good should not be excessive. This is easier said than done. Many fish friends buy back their favorite varieties and make the fish tank full of fish. Although there are many kinds, it is chaotic, and the water quality is turbid, and the fish is short of oxygen, which makes the fish and viewers uncomfortable. Generally speaking, it is advisable for fish to live normally without floating head without oxygen and filtration facilities. It won't be boring, and it will have a natural taste in appearance. Of course, the smaller the density, the better the living environment of fish, and the less difficult it is to raise them.