Single Tank Systems Single tank water softeners are used where water consumption is constant on a day-to-day basis, or when peak demand days are predictable on the same day(s) each week.  Single systems must be designed to meet a minimum of one-days maximum capacity demand and peak gallon per minute (gpm) flow rate required. Multiple Tank Systems Multiple Tank water softeners are used where there is a demand for conditioned water 24 hours a day or at flow rates or capacities higher than those obtainable through a single system. They may also be used where one large system will not fit through an existing doorway.