[jerryclunsford]

Best thing would be plywood across the top then shim bottom to level. Or instead of shims leveling feet.

 

[jerryclunsford]

May just be me but I would have more than 2 screws or nails at each attachment point for your lower top frame to the 4x4 uprights.
you want them good and solid,unlikely to twist.

 

[jerryclunsford]

I just prefer a more fasteners to the possibility of a shattered tank and water everywhere.
like I said, maybe it’s just me.