D.E. (Diatomaceouse Earth) powder is a geological deposit made up of the fossilized skeletons and tests of siliceous marine and fresh water organisms, particularly diatoms and other algae. These skeletons are made of hydrated amorphous silica or opal. When crushed, they break up into tiny pieces until the material feels like talcum powder. This powder, with billions of micropores, offers remarkable filtering properties when used in an Aquarium Products System 1 diatomaceous filter