Abstract: A water monitor for a toilet flush tank includes a sensing system and an alarm system. The sensing system includes a tube having a constricted aperture in the base. The tube is normally filled with water. When the flush tank is emptied water begins to drain slowly from the tube. A float in the control tube responds to the water level in the tube. A contact attached to the float closes contacts on the control tube if the flush tank does not refill. The closing of the contact is used to trigger an alarm signal. The monitor includes a self cleaning drainage aperture for the control tube and a flexible mounting belt which provides for simple mounting and adjustment.