Diatomaceous earth (DE)

Diatomaceous earth is a powder made from the fossilized skeletons of ancient diatoms (single-celled algae), coated onto the grid elements inside a DE filter to form the actual filtering surface. Its irregular, porous structure traps particles as small as a few microns, finer than sand or cartridge filters typically catch, which is why DE pools often look the clearest. It needs to be recharged after every backwash and handled with care (a dust mask is worth wearing) since inhaling it repeatedly isn't good for your lungs.