GJB, you didn't even need my advice, you knew already what to do! I believe that that is the next step, but I'm not a 'stain expert' (mbar and smallpooldad seem to really know this stuff, as do others). I'd look through the 'stains' area and see what can't be culled from an advanced search. I think that sequesterant then ascorbic acid directly applied can be done for small areas without having to treat the entire pool with ascorbic nor changing the cl nor pH, but please check to make sure that's correct. I'm proud that you knew what to do and somewhat abashed that I can't help further. Best of luck with this!