Abstract: A nozzle/venturi with a bypass for controlling the pressure differential and therefore the amount of air which is entrained into the fluid stream. The nozzle/venturi assembly is utilized in a filtering system for removing contaminants from a water supply. The nozzle/venturi assembly includes a nozzle/venturi with a water inlet, an air inlet and an outlet passage and a bypass valve with an adjustable passageway to control the fluid flow through the bypass. The bypass valve includes a diaphragm which is effected by the downstream pressure resulting in an adjustment of the pressure differential and a spring biased seal valve to control the pressure which will open the bypass. The nozzle/venturi assembly provides efficient and precise control of the pressure differential to ensure that a constant amount of air is entrained into the fluid stream through the entire pump cycle.