A water softener comprising two water softener components, 1, 2, valve means 11, a water driven flow-meter 16 for controlling the valve means to cause the water softener components to be rendered inoperable to soften water at respective different intervals so that at least one water softener component is operable all the time, the water driven flow-meter measuring a flow of water such that each time a predetermined quantity of water has been measured, regeneration of one of the tanks is initiated, an adjustment mechanism for adjusting an amount of the predetermined quantity of water required to be measured to initiate regeneration of one of the tanks, and a regeneration system 23 for alternately regenerating the two water softener components. A water softener comprising one water softener component. A fluid driven displacement device comprising a chamber for receiving a flow of fluid, a first mechanism driven by the flow of fluid for detecting and/or metering an amount of fluid flowing through the chamber, a second mechanism driven by the first mechanism for triggering an event in an event in response to the detection and/or metering and an adjuster system for varying the selected amount of fluid.