I have had my pool for six years now. I have learned so much from this site. In fact, I have never had a problem I couldn't get solved by reading the forum. Now I am wondering when I need to change sand in my filter. I have cleaned the sand in the fall each year with the product recommended by the dealer. But my sand level seems to be getting lower. And it looks really gross. Also, why do pool owners add DE to the sand? What is it and what does it do? Thanks, Dorie