I add DE to my sand filter.
My starting pressure has been 14.5 I was able to go four weeks without back washing. Last week I had to backwash and my starting pressure was 15. I back washed a couple of times and couldn't get it any lower. So I figured this is my new pressure. I ended up back washing three times last week because the pressure kept shooting up within a few hours to a day later. I said forget the DE after the last backwash on Friday and last night I thought the pool had a lot more particles than usual floating past the light. So I added a cup and a half of DE. Usually I add two cups, I left the pump running all night and this morning the pressure was up to 19. Why was I able to go four weeks and now I can only go hours to maybe a day, day and a half?
The water has cooled down so no one is even swimming for the last week. The pool is perfectly balanced and looks great.