A humidifier including a base defining a liquid reservoir; a humidification mechanism for dispensing liquid contained in the reservoir; and a tank removably mounted on the base and having a bottom wall including a tube projecting into the reservoir and with a lower lip defining a fill opening, and a liquid discharge opening for providing a controlled discharge of liquid so as to maintain a given level of liquid in the reservoir. Also included is a cap engaged with the tube and having a cover portion closing the fill opening, and an annular gripping portion extending outwardly from the cover portion by the lip and upwardly adjacent to an outer wall of the tube so as to define therewith an annular cavity, and wherein the gripping portion defines control means providing liquid communication between a lower portion of the annular cavity and the reservoir. The gripping portion facilitates removal and installation of the cap while the control means prevents over filling of the reservoir in the event of inadvertent leakage between the cap and tube.