Easy method to figure out how much sand you want: Divide the tank volume by its height to get how many gallons it takes to fill it an inch deep. Then decide how many inches deep you want the sand and multiply it by the first number. Then divide by 3.75 (the number of gallons of sand in a 50 lb. bag) to see how many bags you need. So, if your tank is 30" high and you want 2" deep sand,
((180/30)*2)/3.75=3.2 bags.
Round up to 4 bags and there you are!