Abstract: The invention relates to an apparatus for the gas treatment of mains water in the inflow of a domestic fitting (16), having a connection (12) for a gas source, and having a device (8) for supplying the gas into the mains-water stream. The invention provides the following features: a. the apparatus is designed as a unit which is capable of forming part of the substructure and has a watery inlet, a gas inlet and an outlet for the water or gas-treated water, b. it has a pressure sensor (4) which is assigned to the water main and is designed for emitting a first control signal when the water pressure drops below a predetermined threshold value, c. it has a control unit which is intended for releasing the gas supply and is designed for activating the device for supplying the gas into the mains-water stream as soon as it has received the first control signal from the pressure sensor and an additional, second control signal.