The Fixed Supports are introducing an artificial stress concentration into the model that is not present in the real world.
In reality, what supports the top and bottom of the tank?
There are methods of creating constraints to allow Static Structural to solve without introducing artificial stress concentrations.
For example, a Remote Displacement, Behavior = Flexible, could be used on the bottom circle. All six DOF should be set to zero.
Why do you need a Fixed Support on the top of the tank? Try solving with that suppressed.
Displacement will converge in a model that includes a Stress Singularity. Stress will not converge in such a model. If you are interested in stress, you must do the mesh refinement study on stress.
You could create a 2D Axisymmetric model and have elements through the thickness of the walls to see detailed stress concentrations if the geometry is axisymmetric.