I've been fighting this problem since we opened the pool back in early June. About two weeks after opening, I had it looking pretty good, but not sparkling. Then it rained every day for about three weeks, during which time the water greened up considerably. Once the rain quit, I started shocking and brushing, and after about a week it was starting to look better. A friend of my wife's, who takes care of the pool at her work, came by to look at it a few times. About the time I thought I was getting somewhere, I started getting brown "crud" on the floor, and he said it was sand. So...I changed the sand (the sand in the filter was play sand, which I replaced with filter sand), ran the pump for three days with no issues, then my wife's friend used ascorbic acid to try to get the stains off of the vinyl liner. Two days later he shocked it, and a day after that the "crud" was back on the pool floor. He says it is sand, but I vacuumed last night and I'm pretty sure it's not sand (it "poofs" up when I get the vacuum head even close to it). Now I'm confused, disgusted, mad, all of the above! Someone please chime in with some opinions. By the way, my numbers are: FC 5, TC 5, Ph 7.4, TA 100. Last time I checked CYA it was 30 (about three weeks ago), and the others (CH, TDS) were fine. Sorry for rambling, but I need help!