Well, you will figure the topsoil is 95% of the volume, so 6' x 2' x 2" is 2 cubic feet, or 15 gallons, or 60 quarts, or 60 liters or 200 lbs.
You might find bagged soils sold in any of those units.
Pure soil, that is, sand-silt-clay weighs about 100 lbs per cubic foot.
Potting soil blends have more organic matter, more compost and will be lighter.
All the other ingredients you add to the soil, put together might be 5% (most of that is the clay)
Another 1" of gravel = about 100 lbs, 1 cubic foot. Look for this in a rock yard, masonry store, landscape center or similar place. These stores sell by the truckload or by the bag. You may have to ask what size the bags are. Most gravel weighs about 100 lbs per cubic foot, though there are some lighter ones (like Lava rock).
I know me and i'll go and buy way too much stuff!
You are not buying way too much, you are getting ready for another tank!