Rob
Ansys Employee

In Fluent we use "solid" zones too, we just call them fluids. If you create one zone that is the volume of the tank and a second one for the outer domain you can label the surfaces you need to be walls. Have a look in the Help system tutorials to get some more ideas. In Fluent we can use thin walls (not dissimilar to a shell) provided there are fluid (volume) cells on one or both sides.  


Overall the geometry & mesh parts are very similar, it's just a case of applying the CFD terms and having a decent mesh.