If the water quality is polluted by construction waste or non-degradable domestic waste, no fish can purify the water quality. If there are too many aquatic plants and microorganisms, they can be purified by biological means. For example, herring, grass carp, silver carp, bighead carp, snakehead and Baihe carp are all filter-feeding and herbivorous species. They feed on plankton, algae, aquatic plants and organic debris, which can purify water. There is also a kind of fish "scavenger", the scientific name is "Japanese catfish", which is also called miscellaneous fish, sucker fish and anglerfish. It can absorb the residual bait of algae and sediment, slow down the deterioration of water quality, but it can't purify water quality. Suitable for keeping in an aquarium to clean up the garbage generated by other fish. Scavengers belong to Carassidae family, which are native to rivers all over South America and can survive almost as long as there is water. Scavengers are omnivorous fish, which often adsorb on the stones and glass of aquarium and suck algae, benthos and garbage in the water. Moreover, scavengers don't eat fish feed, and the swimming place is particularly clean and can be mixed with healthy fish species, so they get the title of "scavengers".