An automatic valving unit for a pressure vessel that holds a cartridge, loose media, and the like for conditioning and/or purifying a fluid, comprises a valve head having a first passageway extending through it and being adapted to be connected to a fluid transmission line, an outlet port formed at an outlet end portion of the first passageway in the valve head, a first valve member positioned in the first passageway of the valve head for blocking flow of fluid through the first passageway of the valve head when the pressure vessel is not connected to the valve head, a fluid inlet member formed on the pressure vessel for providing a passageway for fluid into the pressure vessel, said fluid inlet member including a first valve actuating member for opening the first valve member when the pressure vessel is connected to the valve head, and connecting/disconnecting members formed on the outlet port of the valve head and on the fluid inlet member of the pressure vessel for connecting the pressure vessel to the valve head, when desired, to permit flow of fluid from the first passageway of the valve head to the fluid inlet member of the pressure vessel, and for disconnecting the pressure vessel from the valve head, when desired, to stop flow of fluid from the first passageway of the valve head.