PURPOSE: To lessen the trouble of the aquarium cleaning work most important for the maintenance of the health of fish by efficiently filtering residual feed and excreta floating in the water.

CONSTITUTION: The main body 1 of an aquarium is provided with a filtering means 2 to temporarily lift the water in the aquarium from the aquarium main body 1 and filtrate the water with a filter 8, etc., and provided with a diffusing means 4 to drop the water filtered with the filtering means 2 into a diffusion box 11 placed under gravel 3 laid on the bottom of the aquarium main body 1, transfer the water through the gravel 3 via a water-releasing hole 12 opened on the diffusion box 11 and diffusing and discharging again into the aquarium main body 1. As necessary, an air-supplying means 16 is equipped to supply air through an air channel formed in the diffusion box 11, pass the air through the gravel 3 together with the filtered water and discharge into the aquarium main body 1 in the form of bubbles.