Hey @whuaman welcome, I am sure someone else is better to explain this than me, but I am still going to try to make it
Whenever we perform ‘conventional’ CFD, we either conduct internal or external flow simulations or a combination. What this implies is the following:
Suppose we wish to simulate an internal flow simulation (as in your case), we import a certain geometry and first we will make a mesh of the volume trapped by that imported body. And in reality we do not consider the thickness of the tank wall, just the volume trapped inside the solid body that we imported. We need to specify ourselves what the walls of the body do, are they actual physical walls? Or inlets? Etc. Why do you need the thickness of the walls?
Whenever we simulate external flow, the flow around an object is simulated and hence a mesh around that object is created.
The combination is when we would for example have some propeller inside your tank/vessel, then the flow inside the tank is considered, however at the same time it is the flow around the propeller.
If you could specify what you are trying to achieve, maybe I can help you better.