Adding sand

[asylumdown]

Looking through old photos it's become pretty clear that my tank has lost a significant amount of sand. Surprising considering that for various reasons I've vacuumed out maybe 10 pounds of the stuff over the past year but if you look at the images it looks like I've lost nearly an entire 40 pound bag.

Does anyone here ever addd new sand to their setup? Is it normal for that much to dissolve in 16 months?