I just did an Iron stain removal on a pool, and I'm blown away by the results. I used a product called Stain Free, which is Citric Acid. They make a more potent version of the product which uses Ascorbic Acid, but I went with what was readily available to me. The pool started off with really bad iron stains all over the entire liner. The shallow end was by far the worst, so that is where I took pictures. Here are the before and after photos. (The before picture was actually taken about 1 minute after adding the Stain Free. The lighter area on the left was already starting to improve when I took the picture.)
Before:
After:
Before:
After: