You don't apply a force in addition to the hydrostatic pressure. You only apply the hydrostatic pressure load to the bottom and sides of the tank to represent the forces the water puts on the tank. The hydrostatic pressure creates a downward force by acting on the surface area of the bottom of the tank.

For the wind load, you could compute the pressure on the sides of the tank from a CFD simulation in Fluent, or you could look up standards that civil engineers use for wind loads on buildings.