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.
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.