Water fitting for mixing treated drinking water with raw water in the form of untreated drinking water for providing drinking water with a defined hardness, comprising (a) a housing with an inlet configured to be connected to a drinking water supply and an outlet for providing drinking water with a defined hardness; (b) a water treatment unit with ion exchange granules connected to an outlet connection and an inlet connection; (c) a mixing device for mixing treated drinking water with untreated raw water with adjustment means (126, 134) for adjusting the amount of treated drinking water which is added; wherein (d) the mixing device comprises a housing socket at the housing rotatingly accommodating a mixing piston which is adapted to establish a bypass connection between the inlet and the outlet bypassing the water treatment unit; wherein the mixing piston comprises two conical openings which define the flow cross section and thereby the flow volume of the raw water which is added to the treated water; (e) in addition to the inlet and the outlet the housing is provided with an additional inlet connection and an additional outlet connection adapted to have a filter or a pressure reducer filter combination connected thereto; wherein (f) the additional outlet connection and the additional inlet connection for connecting to a filter or a pressure reducer filter combination is formed by a flange where one of the connections is formed by a central channel and the respective other one of the connections is an annular channel concentrically arranged around it, and which is configured to be releasably connected to a corresponding flange of a filter or a pressure reducer filter combination, and (g) the outlet connection and the inlet connection for connecting the water treatment unit are formed by a connection socket having an inside which directly ends in an annular channel (42, 44); characterized by (h) a flow meter in the water treatment device for providing information about the flow volume of the raw water, (i) a control- and processor unit integrated into the water treatment unit; (j) a conductivity sensor with means for transmitting the signals generated by the conductivity sensor to the control- and processor unit, and (k) a sealing sealing in an axial and in a radial direction of the mixing piston, which is inserted into the housing socket with the mixing piston, whereby the mixing device can be fully closed for determining the degree of exhaustion of the ion exchanger granules. (a) a housing with an inlet configured to be connected to a drinking water supply and an outlet for providing drinking water with a defined hardness; (b) a water treatment unit with ion exchange granules connected to an outlet connection and an inlet connection; (c) a mixing device for mixing treated drinking water with untreated raw water with adjustment means (126, 134) for adjusting the amount of treated drinking water which is added; wherein (d) the mixing device comprises a housing socket at the housing rotatingly accommodating a mixing piston which is adapted to establish a bypass connection between the inlet and the outlet bypassing the water treatment unit; wherein the mixing piston comprises two conical openings which define the flow cross section and thereby the flow volume of the raw water which is added to the treated water; (e) in addition to the inlet and the outlet the housing is provided with an additional inlet connection and an additional outlet connection adapted to have a filter or a pressure reducer filter combination connected thereto; wherein (f) the additional outlet connection and the additional inlet connection for connecting to a filter or a pressure reducer filter combination is formed by a flange where one of the connections is formed by a central channel and the respective other one of the connections is an annular channel concentrically arranged around it, and which is configured to be releasably connected to a corresponding flange of a filter or a pressure reducer filter combination, and (g) the outlet connection and the inlet connection for connecting the water treatment unit are formed by a connection socket having an inside which directly ends in an annular channel (42, 44); characterized by (h) a flow meter in the water treatment device for providing information about the flow volume of the raw water, (i) a control- and processor unit integrated into the water treatment unit; (j) a conductivity sensor with means for transmitting the signals generated by the conductivity sensor to the control- and processor unit, and (k) a sealing sealing in an axial and in a radial direction of the mixing piston, which is inserted into the housing socket with the mixing piston, whereby the mixing device can be fully closed for determining the degree of exhaustion of the ion exchanger granules.