After battling a brown, metal-stained fiberglass pool every summer for the past four or five years, I thought I finally had it licked this spring. I started off with an ascorbic acid treatment and Jack's Magic purple (SWG). The pool looked great for a few weeks. Every time the pool walls started looking a little stained, I added another bottle of Jack's and the stains gradually disappeared. Well for the past week, the pool walls have turned various shades of brown. Using a vitamin C tablet, I've confirmed that the stains are metal. Using Jack's Magic Sequest Test Kit, my sequestrant level was 14 ppm, and I added another bottle to bump it up to 20 ppm, but the stains still persist. I use municipal water to top off my pool, and the area is known for soft water.
Here's my current pool chemistry:
FC = 5
pH = 7.2
TA = 80
CH = 140 (Just added to try to bump this back to the 200's)
CYA = 70
Sequestrant = 20 ppm
I called the company that made and installed my pool. They tell me that the stains are the result of chlorine burn (their term, not mine). They insist that FC should be kept at the bare minimum of 0.3 - 0.5 ppm and pH at 7.2 - 7.5. When I told them that my FC was 5, they insisted that this was the root of my problem. Has anyone ever heard of chlorine burn on fiberglass pool walls? Regardless, does anyone have any recommendations? I have an iron test kit on order, and I plan on trying ferri tabs in the near future.