DE is a type of filter media. It stands for Diatomaceous Earth and is a porous white powder that is used in some pool filters. It can filter out very tiny particles and really cleans the water. It does do a great job, but in my opinion, is not as easy as maintaining a sand filter. Several sand filter owners like me do routinely add a very small amount of DE to their sand filters to help it catch the really small stuff that the sand wouldn't filter out alone. If you do this little trick, however, only add a very small amount. Only enough to raise your filter pressure by 1psi. I think I usually only add about a half a cup. (I have a 24 ft. AG pool with a 200 lb. sand filter.) The pressure rise isn't noticed immediately, so wait a bit after you add it before deciding to add more. If you do happen to add too much and cause too much of a pressure rise, you can always backwash your filter.