Abstract: A water treatment system (1) for a reservoir (3) of water (5) including a mixing unit (2) located outside of the reservoir (3) and in an overall flow path of water from the reservoir (3) to and through the mixing unit (2) and back into the reservoir water (5). The mixing unit (2) is operable in three disinfectant modes that selectively add (a) chlorine, (b) ammonia, or (c) a blended mixture of chlorine and ammonia forming chloramines into the water returning to the reservoir (3). The mixing unit (2) also has hard and soft flush modes. In the hard flush mode, water (5) from the reservoir (3) is continually moved to flush through the mixing unit (2) and back to the reservoir (3). In the soft flush mode, the incoming hard water (5) from the reservoir (3) is softened to remove calcium and other minerals before passing through the mixing unit (2) and back to the reservoir (3) to reduce the undesirable build up of mineral deposits in the mixing unit (2).