Enter the size of your aquarium and how far the water is below the top of the glass. The calculator shows how far to lower the water line to change exactly the amount you want.
Enter the length, width and depth of your aquarium to see it drawn here.
Volumes are based on the glass dimensions of an empty tank. Substrate, rocks and decoration take up some space, so the actual amount of water is a little lower. The level drop is accurate as long as little decoration sits at water level.