I neglected substrate weight in my calculations, but it doesn't make much of a difference. The weight of water is 62.4 lb/ft^3... Depending on the soil, it could be anywhere from 55-100 lb/ft^3. Most calculations I have seen, say for a 55 gallon, calculate the weight of 55 gallons of water, then the weight of the substrate, and add them together. This is not accurate, since the substrate displaces an equal amount of water. So you would only add the difference in the weight of the water and soil. Say the soil is 100 lbs/ft^3, so you only add 37.6 lbs per cubic foot of soil added. In a 55 gallon tank (4ft x 1ft) with 3 inches of substrate (1 ft^3), you're looking at 462lbs of just water, or 500 lbs with substrate. The extra ~40lbs is negligible.