Well, I measured things today. It looks like I'm going to need quite a bit of fill just as I thought. The edge of my yard is 3.5 ft from the basement floor as shown in the picture below. I had planned on the tank being about 4.5ft tall. Then, you have to add the slope for the drain back piping on top of that. 80ft at .25 inches per foot gives us 20 additional inches. So, it looks like I'm short about 2.5feet at the edge of my lawn. Plus, you need to have a few inches of dirt over that, so lets just say 3 feet. That sounds like a LOT of fill...