The pool does not test for the presence of metals. I too had little expectation of the vitamin C doing anything (yes its ascorbic acid), just wanted to rule it out.

My current hypothesis is that its organic in nature. I dropped some chlorine in one shallow area of the spot, and it cleaned up. The area near it had been yellow too, and it changed its hue to more of a gray. It looks like its just dirty there, but of course does not brush up.

I brought a water sample in for testing. No metals were seen. But phosphates, TDS, and cyanuric acid are all pretty high. I think my next step is going to be a partial drain and refill before I try to just attack with a bunch of chemicals. After partial refill, I'll test again and shock if looks like organic based.

I'm a bit perplexed about the high phosphates. I routinely use "Jack's Magic The Pink Stuff" as a sequestrant, as I have had iron stains in the past. The label really doesn't go into the details of it, except that its not acidic. Does anyone know if it adds phosphates ? (I'd probably use it even if it did, as its been good for me. I just want to understand any side effects it may have.)

If anyone has any other good ideas, I'd love to hear them. Thanks.