I have 4 year old pool, 18,000 gallons, salt water system with Chlorine generator, white plaster, in Northern California, a area noted for "hard" water. 2nd summer noticed grey stains, along with an orange / rust colored stain. 2 summer's ago grey stains got bad, orange stain is worse also. Tried 2 bottles of stain remover, no luck. Sanded pool, removed most, not all of stains, but stains quickly returned. Last fall and this spring added apporoximately 20 bottles (over a period of months) of stain remover. I used a PVC pipe to get stain remover right on worse stained areas, and this temporarily improves stains, but stains come right back. Pool builder, "leaves, not brushing enough, your city's water (I had the city test my water, the city's water company strongly, in no uncertain terms disputed this: I discussed with builder and never heard again about my city's water) "lawn fertilizer in pool", we even tried a phosphorus remover, Pool builder added a inline "metal remover". Point is that contractor while verbally supportive, really doesn't seem to know what to do. I keep the pool very clean, chemical tests at builders store are always correct, (I have been instructed recently by pool builder "to keep ph at 7.2, for a plaster pool") never had an algae problem. Calcium levels OK, wore out brushes, tried a stainless steel brush, sanded to the point that the plaster feels as smooth as any pool I have been in. I do have a scale problem for which I routinely add scale and stain remover. Last couple of years I haven't been diligent about brushing, but if sanding didn't fix? I came across an article from a Pool builders convention in Las Vegas that basically implies that stains are / maybe inevitable with salt and plaster. Next step is consulting a professional, already invested lots of money and way to much time. Hoping someone has a solution. raiderron