Dosing chambers, for use with pressurized water filters of the type used with ponds, require isolation before accessing to prevent water egress. The invention comprises a chamber in which dosing substances may be placed, a water inlet and a water outlet, a lid for sealingly closing the chamber, a valve for closing the water inlet and an interlock means for operating the valve to close the water inlet as the lid is removed, so that when dosing substances are placed into the chamber pressurized water is prevented from issuing through the inlet.