[ul]
[li]Assume the soil is water.[/li]
[li]The volume of the tank as equivalent water becomes the buoyant force.[/li]
[li]Subtract the weight of the empty tank.[/li]
[/ul]
The difference defines the concrete slab needed to keep the tank under. Don't forget to add concrete to offset the buoyancy of the concrete, size the straps for the load and apply safety factor.
Due to degradation of our industry, most tank manufactures have this canned and often provide counter buoyancy packages.