I'm currently adding DE to my sand filter (Thanks Carl and Al!!!) with excellent results. I also run my Dolphin every week. Once cycle to clean the pool, then another cycle to "polish" the water. I must say that between last year and this year, it has made a HUGE difference in the water clarity when I have the light on at night.

By the way, my parents have run a sand filter for at least 10 years and have never replaced the sand. Pool still looks as good as ever.