How much ascorbic acid do you have? You may want to try the sequestering agent first, because you need to put it in anyway, even if you do the treatment. I would lower the ph down to 7 - 7.2 add the sequesterer - make sure you use enough - more is better than not enough. If after 24 hours you don't see the stain lighten, then use the ascorbic acid, you probably won't need as much as you used the first time. Just bring your chlorine level down, and pour the ascorbic acid where the stain is the most noticable. Let us know how it goes