At what pressure do the heads deliver that gph? Your storage tank's pressure and volume will decrease as water is drawn from it. The draw down from a storage tank is directly related to its initial air pressure charge. If you precharge it to a level just below the pump's turn on pressure, it will be "empty" shortly after the pump starts if it cannot deliver the demand from the sprinkler head, which yours cannot. As your precharge pressure is reduced, more water can be stored in the tank, but it will not be released until the pressure in the system degrades due to an excessive draw, and at the same time the system pressure will also be reduced to the level of the residual air charge in the tank at that point, so the sprinkler heads efficiency will also suffer. In order to have 210 gallons at 14 gpm and at any acceptable pressure, your storage tank may have to be in the neighborhood of 1,000 gallons.