IMHO, 1 lb of ascorbic acid will treat 10000 gal...therefore you needed 1.5 lbs initially. Most of the metal sequesterants I have seen also treat 10000 gal with 32 oz so you would have needed 1.5 qts. initially.
Depending on the stain it might take a few days of filtering to remove all of it. If your water is cloudy that is usually a good sign.
Did you test the stain with a vitamin C to see if it would remove it? Not all stains will be removed by only ascorbic acid...sometimes more involved procedures are needed.