PURPOSE: To provide a cleaning method for a water tank for aquarium fish comprising simple structure, capable of preventing water turbidity in a water tank for aquarium fish and offering sufficient irradiation effect with ultraviolet light.

CONSTITUTION: A water raising route by a foam group generated by air supply from the outside is formed by using a cylindrical case 1 provided with an air diffusing member 6 at the lower part. Since a discharging tube 7 of an ultraviolet lamp is installed in an exposed state in the case with a small gas between the inner wall and the tube so as to generate ozone by bringing the air foam group into contact with the discharge tube, effective cleaning action on water occurs by sterilization through generation of the ozone and irradiation with ultraviolet light.