Let me explain a little bit about the water tank supplying the sprinkler system. Once the sprinkler system is activated and draws off water from the tank, the fire department will then respond within a certain time element and hook up their pumper connections between fire hydrants supplied by the tank, fire engine and building. As the firemen enter the building, presumably not engulfed in fire, with their hoses flowing or not flowing, in all likely hood they'll turn off the sprinkler system and attack the fire. A lot more water may or may not be needed, therefore, your water tank may or may not have an insatiable desire for more water. An external dependable water source to the tank is crucial. All of these sequences along with sprinkler water demand and duration of that demand will dictate the size of the tank and then some for good measure.