The image below is the line-by-line computation of the mass of water causing a pressure on the bottom of the tank that the hydrostatic pressure load will apply to the bottom of the tank.

The legs of the tank will see the downward force from the pressure on the bottom of the tank as if there were water in the tank. The Force is the Weight of the water. That is included in the model unless you don't create a bottom surface for the tank. If there is no bottom surface on the tank, there will be no downward force.