Thanks JohnT....I do think the sand level is about at that point. We just replaced the sand about a week ago and were told to put that much....we were not totally sure what size our filter was when at the pool store, purchasing the sand. I do know that we put in 250 lbs and I just saw in my "In the swim" mail/circular that a 18" sand filter is used for up to 16,800 gallon pool size...which leads me to believe mine is a 18" because it has 13,500 gallons.....Anyway, just curious if this would affect being able to filter out the green??? However, my pressure gauge rarely ever increases to indicated a backwash...