JasonLion said:Ascorbic acid is way way way milder than an acid wash. There won't be any plaster etching, though the metals coming off can change the texture of the surface.
As far as the filter goes, normally any iron in the filter would be removed. However, there are rare cases where something else happens. With a lot of iron in the water, stains can form very quickly under the right conditions. Remember, this isn't at all common. The whole goal of the treatment is to avoid those conditions. However, if something goes seriously wrong, you can deposit iron very quickly. If that happens inside the filter, the filter media can be destroyed. If that happens just about anywhere else then you are back where you started, which isn't good but also isn't terrible.
This is just the information I was looking for. I ordered some AA from 3 weeks ago and removed my stain. Then I put in some sequestrant. My pH was consistently around 7.2, but 2 days ago, the stains came back out of nowhere. I checked the pH and it was 7.6. So now I guess it's back to the drawing board :|