The Grp water tank capacity is the amount of water that can be stored in a tank when it is at its maximum size. The amount of water stored in a tank will depend on several factors, including the size of the tank, the surface area of the tank, and the water pressure in the tank. How much water your tank can store depends on a number of factors, including the tank’s size, the tank, the surface area of the tank, and the water pressure in the tank.