After replastering pool seven years ago in this southwestern desert town, noticed an insidious, yellowing "bleed-through" this past year which is totally erasing all my bright white, relatively new marbleite. Something is taking over the entire smooth surface with no change in texture, like crusty calcium feel or something. Year round, I've kept chemical balance perfect (pH 7.4 avg; TA 80 ppm; chlorine within range and on the high side during swimming season, and regularly shocked; CYA in range; borate within range; phosphate <100), checked twice weekly, walls brushed weekly, and water drained when CYA goes too high. Some of us got nothin' ta do, right, but what is this stubborn, out-of-nowhere Crud? Last draining was about a month ago, and a stiff bristled nylon brush wouldn't do anything but 150-grit wet/dry sandpaper did the trick, took it all off with no change to plaster texture. I treated it with absorbic acid, nuthin'. Treated it for metals (Metalfree product), same thing. Algaecide treatment for the first time ever and ditto, nuthin'. This week I'm trying Phosfree for the first time ever even though phosphate level low, probably another waste. I haven't tried the dry acid in a sock trick, yet, because I want to know the cause of this phenomenon. As you know, an acid wash erases everything and it's starting to look like a complete wet sanding (polishing?) would be better anyway. Thoughts? Thanks...JS