Diatomaceous Earth (D.E.) Filters are filtration systems that can be used with all types of pool systems, both inground and aboveground. D.E. is composed of microscopic skeletons of Diatoms, in the form of dust or powder. The D.E. is used to coat your filters grids and act as a barrier, or filter, for the dirty water passing through. D.E. filters are very popular filter systems because they are inexpensive to run and can filter down to 3 to 5 microns, which is much smaller than what the naked eye can see.