Abstract: An add-on liquid overflow shut-off valve for a tank. The device is used to ensure that a holding tank is not overfilled and provides a fail safe way to prevent spillage when pumping into a holding tank. The device provides a warning indicator when the holding tank is near full. The device also has a fail safe switch which will turn off air to a pump when the holding tank is full and also blow a whistle to indicate the tank is full. The device uses a diaphragm which operates on a pressure differential. When the pressure is increase past a spring set point, the diaphragm raises a plate that activates a switching valve which shuts off air supply to an air pump and blows a whistle.