Why do experts advise keeping shellfish in an aquarium?

An aquarium is a small reservoir in a glass container, which is located in a house, in an apartment. The inhabitants of the aquariums are plants and different types of fish. From plants – this is Javanese moss, vallisneria, arrowhead. Fish – guppies, scalars, swordtails, barbs and others. There must be shellfish in the aquarium. For example, ampullary snails. They help to clean the walls of the aquarium of green deposits, and also eat the remnants of food that the fish did not eat.