Is a 6 inch reinforced slab suitable for a 12,000 lb storage tank?
Geometry time. 6 foot diameter is 3 foot radius is 9π square feet.

Ground loading for 12,000 lbs spread over that area is 425 lbs per square foot (rounded up.) That's not a concern on almost any soil, nor any grade of concrete - it's a very low loading. 1000 lbs per square foot is typically assumed to be fine for even poor (loading-wise) soils.