Concrete slab const. relating to weight

You would calculate the required thickness and rebar spacing based on the most restrictive failure condition. In your case, you need to evaluate diagonal shear in the concrete, punching shear through the concrete, and direct compression failure of the concrete. Since it is a slab, you also need to look at soil bearing capacity, and the potential for tension failure of the slab if voids develop under the slab and the concrete needs to bridge the gap.

The actual loading depends on the specific details of the water tank support, i.e. is it on feet, or is it in a cradle, or is it supported some other way.

By the way, slab design for heavy loads is what structural engineers get paid to do, but if you want to do it yourself, those are some of the design parameters you need to look at.

Depending on the size of the heavy equipment, you may be better off supporting it on a separate foundation (different thickness and amount of reinforcement than the rest of the slab) and locate it to create joints in the normal slab to prevent cracking. - It all depends on where the legs (or loads are).

The alternate is to over-build the entire slab and place the load anywhere you want to. A 10x15 slab at 6" thick only takes 3 yards of concrete, which is the minimum in most areas of the world.

The 7000# is not that much load if it and its location are defined. For that load, you only need about 4-6 square feet maximum with decent soil.

It is difficult to save even pennies on a small job.

Dick