DE is diatomaceous earth--it's actually ground up skeletal pieces of an algae-like organism. It's ground up into almost a powder, and it's the filter media used in DE filters, which filter smaller particles than sand does. However, you can use a handful of DE in a sand filter to help get the filtering quality of a DE filter but with the ease of a sand filter. It's sold by the bag at pool stores, and possibly at your local hardware store.

Janet