Abstract: An ultraviolet water purifier for destroying harmful water-born algae, bacteria, fungus, yeasts and viruses includes a main receptacle which is molded into a shape representative of a boat and may be mounted to the side wall of an aquarium and an ultraviolet light fixture disposed over the open top of the main receptacle for directing ultraviolet radiation into the water which passes through the main receptacle beneath the ultraviolet light fixture. The main receptacle includes a plurality of various sized apertures and corresponding plug members for sealing the apertures closed. Inasmuch as the various plugs are removable, the selection of which apertures to leave open and which apertures to close depends upon the particular flow pattern through the main receptacle which is desired. The main receptacle is divided into three different chambers with a flow passageway adjacent the bottom surface of the main receptacle communicating between each pair of adjacent chambers.