Did you drop your FC to about 1 ppm and your pH to about 7.0-7.2 before putting in the ascorbic acid? Also, did you put a seqesterant in the water after you put in the ascorbic acid? IF not that could be why some staining still remains. If your pH is high I would try dropping it down and see if that helps. Filter for about 24 hours. If you has high chlorine levels when you put it in you burned some of it up before it got a chance to work on the stains. You might need to get the chlorine and pH down and repeat the treatment in that case.