Uh-h. You need to pick who's helping you on this. You'll end up with the worst of both worlds, going from here, to the store, and back to here.
I think maybe you didn't see some of the email replies. I'm going to add them here. I emailed Chem_Geek about what happened and here are his responses:
andNope. Ascorbic acid is acidic and a reducing agent. The brown turning black sound more like oxidation. I noticed you asked her for the brand — maybe there was something else in the Vitamin C tablet. Either that or maybe the ascorbic acid reduced iron but then there was extra chlorine that re-oxidized it black. Since the ascorbic acid was in a sock, that sounds most logical since it probably wasn’t enough to remove all the chlorine in the pool.
After he sent me those comments, I got your email about the black spot going away AND about the Vitamin C clearing the spot around the tablet. That made things fairly clear: you have iron stains. The vitamin C in a sock reduced the stains, in place, to a black "ferrous" iron state, but that gradually disappeared because ferrous iron soluble. The more direct application of vitamin C behaved in a more typical fashion.Normally, stains don’t reappear after even a local ascorbic acid treatment, but maybe this particular pool has a LOT of iron in it. Probably doing your technique of trying to capture the iron in the filter by raising the pH through the skimmer would work — assuming the filter is a sand filter that could be readily backwashed.
If you're going to follow their program, that's fine. It sound similar to what I would have done, but I'm not sure because I'm just guessing at what the chemicals are. Also, the program they've got you on is likely to allow algae to grow, while you work on iron, and I'm not sure if that's what you want to do.
Regardless, do lower the pH. But, if you lower the chlorine to 1.0 with a CYA of 100 . . . your algae is going to take over, and the pool store should have known that and warned you.
Your choice though.
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