DE is diatomaceous earth which is the ground up fossilized remains of diatoms, tiny aquatic organisms. It is used as the filter media in DE filters among other uses. You can get it at a pool store and maybe at Lowe's or Home Depot, etc.

This thread may be helpful. (Until your registration is complete, you'll probably need to do a cut and paste and paste it in the URL of your browser window AFTER you log out to be able to get to it.)

http://www.poolforum.com/pf2/showthr...to-sand-filter