Hydrostatic Pressure

It is the pressure at a point within a fluid at equilibrium. It is given by

$$\boxed{p=p_0+\rho gh}$$

Derivation

We know that $F_x\&F_z=0$

At Equilibrium (a=0), Forces in $y$ direction is given by

$$\sum Fy=0$$

Collecting all the forces acting vertically

$$-p_o\cdot dA+p\cdot dA-d\text{W}=0$$

Weight acting on the Fluid volume

$$-p_o\cdot dA+p\cdot dA-(\rho \cdot V)\cdot g=0$$

Volume as a product of height and area

$$-p_o\cdot dA+p\cdot dA-(\rho \cdot dA\cdot h)\cdot g=0$$

Rearranging terms

$$\boxed{p_o=p-\rho \cdot g\cdot h}$$