Why advertised gallons differ from real water
A tank's nominal size is a marketing label chosen before the glass is cut. The dimensions are exterior, so the interior is smaller by two glass thicknesses in length and width and one in height. Then there is the gap at the top, the substrate, and the rock. A 55-gallon tank that calculates to 56.7 gallons from its outside dimensions will typically hold 48 to 51 gallons of water once it is set up.
Water weight
Water is the heaviest part of an aquarium. Add roughly 10 to 15 pounds per gallon of capacity for a rough total including glass, substrate and rock, and remember that floors are rated per square foot. A 180-gallon tank on a 72 × 24 inch stand puts around 2,000 pounds on 12 square feet.
How to use this table
Use the water column for dosing and water changes, and the weight columns when checking a stand or floor. If your tank is not on the list, the rectangular calculator accepts any dimensions and lets you enter the real fill height.