Abstract: Coupler/diverter has housing with an aerator at the bottom and an inner barrel having rotary valving stem serving to distribute supply water, brine and overflow from RO filter. Depending on the setting, the stem permits flow through the housing to issue in the usual aerated stream. At this setting it also shuts off all other flow for decoupling. In diverting setting it sends supply to filter, shuttles return brine flush to the outside of the housing and lets reservoir overflow pass through the aerator. In a working setting the stem also can control volume of brine return to a varying degree to optimize the operation of the RO filter.