We've heard good things about the CF rather than DE. The big issue seems to be expense--DE is really cheap and CF is not.

I do not know if you can add CF to a sand filter the way you can add DE to it, but the amount of DE you add is very small (1/3c to 1 cup) per backwash for strikingly good results. After a season and a half I've not quite gone through the first 5 lbs. Since I have another 15 lbs, it will be several seasons before it's worth looking into.

However, one place where CF is the ONLY choice is if you want to improve the filtering on a cartridge-based system. I gather it's safe to use a little bit of CF to improve filter, but you must NOT use DE for that application. It will clog the cart, possibly ruining it.

DE can also be used for ant control, sprinkling it around their nest and where you don't want them.