for s 36x18 foot print
1/2 inch deep = 1.4026 U.S. gallons, 0.1875 cubic feet
1 inch deep = 2.8052 U.S. gallons,0.375 cubic feet
2 inches deep = 5.6104 U.S. gallons,0.75 cubic feet.
3 inches deep 8.4156 U.S. gallons, 1.125 cubic feet.
so when you know what you are going to put down determine its weight per cubic foot of volumes or gallon.
example quartz sand.
weighs 12.84 lbs per gallon
But you dont buy it in gallons right it comes in *Lb bags,
so 50 lb bag would contain 3.89 gallons.
20 Lb would contain 1.6 gallons.
so 2 inches deep of something which Weigh's equal to above would require
) X 13(Lb/gal
) 72.8Lb bag,20 +50 lb bags.
Or 18.2 Lbs for every 1/2 inch of Depth. ( 20 lb would be~9/16" deep.)
the trick is to determine what It weighs by volume..ton/yrd, lbs/gal
Then you can Apply the above Volume examples of Your tank to determine how much to buy.