Method for operating a mains-fitted water filter cartridge having a housing in the form of a pressure vessel, having an inlet and having an outlet for water, characterized in that inside the pressure vessel a main flow (6.1) of water, to which minerals are added, is conducted through a main conduit and a dosing flow (7.1, 9.6) of concentrated salt solution is conducted through a dosing conduit (7.2, 9.5, 9.7), wherein the dosing conduit (7.2) branches off from the main conduit and leads through a storage vessel of constant volume which contains a concentrated salt solution formed of sulfate salt, chloride salt and/or hydrogencarbonate salt, wherein the part (9.7) of the dosing conduit departing from the storage vessel opens into the main flow (6.1) at a dosing point (9.8) via a dosing opening (9.7; 13.2) having a constant flow cross section, and wherein the flow resistance in the main flow (6.1) is set by means of a bed of a granular material (6.3) forming a resistance section (6.5) in the main flow (6.1), which is arranged upstream of the dosing point (9.8) in the direction of flow, and the flow resistance in the dosing flow (7.1, 9.6) is set by means of a separate resistance section (7.5), in the form of a resistance layer (7.4), the feed of which projects into the preceding bed, and/or of a capillary, in the dosing flow (7.1, 9.6), such that a differential pressure arises between the main flow (6.1) and the dosing flow (7.1, 9.6), this differential pressure bringing about a volume flow of the dosing flow (7.1, 9.6) of the salt solution (11; 11'), through the dosing opening (9.7; 13.2) that opens into the main flow, which is substantially proportional to the main flow. inside the pressure vessel a main flow (6.1) of water, to which minerals are added, is conducted through a main conduit and a dosing flow (7.1, 9.6) of concentrated salt solution is conducted through a dosing conduit (7.2, 9.5, 9.7), wherein the dosing conduit (7.2) branches off from the main conduit and leads through a storage vessel of constant volume which contains a concentrated salt solution formed of sulfate salt, chloride salt and/or hydrogencarbonate salt, wherein the part (9.7) of the dosing conduit departing from the storage vessel opens into the main flow (6.1) at a dosing point (9.8) via a dosing opening (9.7; 13.2) having a constant flow cross section, and wherein the flow resistance in the main flow (6.1) is set by means of a bed of a granular material (6.3) forming a resistance section (6.5) in the main flow (6.1), which is arranged upstream of the dosing point (9.8) in the direction of flow, and the flow resistance in the dosing flow (7.1, 9.6) is set by means of a separate resistance section (7.5), in the form of a resistance layer (7.4), the feed of which projects into the preceding bed, and/or of a capillary, in the dosing flow (7.1, 9.6), such that a differential pressure arises between the main flow (6.1) and the dosing flow (7.1, 9.6), this differential pressure bringing about a volume flow of the dosing flow (7.1, 9.6) of the salt solution (11; 11'), through the dosing opening (9.7; 13.2) that opens into the main flow, which is substantially proportional to the main flow.