I am wondering if someone can comment (or point me to a thread) on sand coming out of the return vent. I have a sand filter and have noticed lately that when I run it for a longer period of time like overnight, I get small buildups of brown sand (or fine silt, maybe?) on the floor of the pool. My pool floor will start out clean before running the filter, and then will have the small brown piles. As I say, I am not sure what they are made up of - maybe it isn't sand at all. Could it be dirt or sand so fine that the filter doesn't trap it? Besides that, the water looks great, levels are fine, flow and pressure are good, I backwash as required, etc. Any hints?