Hi Steve: you can directly use the hydrostatic pressure equation because there is an option on the pressure loading dialog to vary the pressure according to depth. I believe it's an "advanced" option when you look in that pressure dialog. You can set up a reference coordinate system at the bottom of the tank, for example, and use that to vary the pressure from max at the bottom to zero at the water line. One equation can be used and you can select the bottom and side walls and it will correctly apply the pressure according to depth below the water line. Hope that helps!