In general, how did the water pressure in the tank change when mass was added to the fluid?

As the height increases the pressure must increase.

Explanation:

When we add masses to the fluid, the amount of fluid in the tank increases, therefore its height increases and the pressure is described by the expression

           P = ρ g h

where rho is constant for a given fluid and h is the height measured from the surface of the fluid.

As the height increases the pressure must increase.