United sells the Oxalic Acid treatment, so that's what you have from the pool store. You need to drop your chlorine down to 0 and lower your Ph to 7 or 7.2 and then do the treatment, that way you will get the best results with it. Expect cloudy water and after 48 hours start bringing your Cl back in gradually. If the Oxalic doesn't work I would have to recommend the Ascorbic Acid treatment and preparation is the same for that treatment as well. Running high Cl will be making your stains stick better so lower it and stop running at shock levels, if the stains aren't organic (algae and such) then it won't take them off and you're making matters worse.

Good luck with it. Let us know how it goes with the United (Oxalic) treatment.