The density of water is 1000kg/m3. Meaning for the 1m3 = 1000kg.

If your tank is for landed unit, most likely your tank size will be around 220igal to 330igal @ 1.0 to 1.5 m3. You need a structure that can hold at least 1.5 to 2.0 times of the worst case load of 1.5 tons.

There should be a flat slab structure near to the staircase area to house the water tank.