Iron is your problem, Oxaylic acid is a known remedy for rust (oxidized iron) stains! It is toxic, however. Phosphonic acids are also effective and are the active ingredient in most of the "metal removers" I have seen. You can learn a lot about a product by going to the manufacturer's website and reading the MSDS for it. Ascorbic acid works (if I am not mistaken) by acting as a reducing agent just like Oxaylic and Phosphonic acids. These will also lower your pH and their effectiveness is dependant on pH so read your directions!