I have a 15000 gallon inground, completely screened in. It is about six months old. I check the water chemistry twice a week and everything has always been perfect and still is. But, starting about two months ago, if I dont brush my pool atleast three times a week I get brownish yellowish dirt buildup stuck to the sides and bottom and every nook and cranny. It brushes right off except for one spot now on my swimout that I actually think is stained from it. I think I may sell my house before the marcite is all stained.