“Plug-in” fluid flow diverters are disclosed. Additionally disclosed is water purification equipment adapted to communicate with the diverters. Such equipment is especially (although not solely) useful in connection with circulating streams of water present in systems associated with swimming pools, hot tubs, and spas, as the circulating nature of the water permits more of its volume ultimately to be diverted into the equipment for purification. The diverters, furthermore, may have entrances oriented to receive fluid travelling in only one direction through a conduit and be keyed to one or more components of the equipment for purposes of identification, inventory control, or otherwise.