No, don’t deactivate the hydrostatic pressure unless you want to see the stress in the tank when it has no liquid, but is pressurized. 

Just leave the hydrostatic pressure active in step 2 and also have a separate pressure load on all the tank wall faces, including the faces below the liquid free surface.

The air pressure is pushing on the free surface and adding a constant pressure offset to the liquid, which transfers it to the wall.