The invention relates to a method and a device for preventing the blocking of the brew group, the filter holder or the coffee filter (8) in coffee machines when using fully softened water for the preparation of coffee. According to the invention, it is proposed to avoid the blocking of the machine by metering calcium and magnesium ions to the fully softened water. According to the invention this is achieved in that a device with a bag or reservoir (4) containing the calcium and magnesium ion containing solution, is positioned in a cartridge (3), which is arranged in the water tank of the coffee machine. The bag is in contact with the fully softened water via a capillary (5), which is closed with a check valve (6).