This stain started two years ago when we started using an intex SWG. It started just below the water line and went down about a foot the first year. This is after another year. I have looked at everything about stains, but haven't seen one just like this. Without water in the pool, I can't test for metals. Could it be copper or iron? From the SWG? My chemistry last year ran about right. I didn't have a way to test for metals.