The pressure in the tank increases as the tank is filled. The gas expands and pushes against the walls of the tank. Because the tank has no walls, the gas pressure in the tank is the same everywhere, so the expansion of the gas is uniform throughout the tank. But The pressure of the gas is highest at the top of the tank, where the pressure of the gas is the greatest.
What causes the gas to expand? Well, An increase in the volume of a gas that is confined to a container causes pressure to rise because the force exerted on the walls by the gas is greater than the force exerted by the walls on the gas. If the walls of the container are fixed, then the gas exerts pressure on the walls of the container and thus increases the pressure inside the container. The volume of a confined gas can be increased by heating the gas or by compressing the gas.
On the other hand, If you put any amount of liquid there in the tank. And then you add Air; the liquid level rises, the air molecules will compress the liquid in the tank. This will make the liquid rise. If the tank is then emptied, the liquid will return to its original position.
