Will the water fill the tank or simply flow through the particles? 

If the water fills the tank, the water will be a larger load than the dry particles. You can simulate the stress in the tank caused be being filled with water using a Hydrostatic Pressure load on the inner surfaces of the tank up to the free surface of the water.

To consider the load caused by particles, you have to know the angle of repose of the material.

The angle also depends on whether the material is dry or wet.

Ansys has a Geomechanical material model called Mohr-Coulomb that is used to model soil.

Ansys Explicit Dynamics can have materials defined by spherical particles that can flow like sand.