An apparatus for maintaining a liquid level within a receptacle such as a Christmas-tree stand water basin or a pet's water dish includes: a) a liquid reservoir adapted to be positioned above a height of a liquid holding basin of the receptacle, where the reservoir includes an open upper end and a lower end, where the open upper end includes a threaded, cylindrical outer surface and the lower end includes a reservoir outlet orifice; b) a cap including a cylindrical wall having an enclosed top end and an open bottom end, where the cylindrical wall includes a threaded inner surface from gauging with a threaded outer surface of the liquid reservoir; c) a valve assembly provided approximate the reservoir outlet orifice of the reservoir and operatively coupled to the cap to open the reservoir outlet orifice when the cap is threaded downwardly on the reservoir to a valve-open height below an actuating height an operative to close the reservoir outlet orifice when the cap is threaded upwardly to a valve-closed height on the reservoir above the actuating height; and d) a circumferential seal provided radially between the reservoir and the cap when the cap is threaded at least between the valve-open height and the valve-closed height.