You are going to have a bit of a chlorine demand because ascorbic acid (and antioxidant) will destroy chlorine (an oxidizer) until all the ascorbic acid has been oxidized! If you have not added any polyquat 60 to help prevent an algae outbreak it's probaby a good idea to do so and introduce the chlorine very slowly. Add enough to raise your FC by no more than about 2 ppm at a time and wait a few hours before additions. At first the chlorine will not hold but after a while (deepening on just how much ascorbic acid you added and just how much got used up in removing the stains) you will begin to see your FC holding. At this point bring your FC up to your normal value (based on your CYA), still at a rate of no more than 2 ppm at a time) and keep it there by adding chlorine as needed (might be a few times a day at first) Once the FC is holding normally you are done.
Do not shock for at least 2 weeks! I havefound that keeping the pH on the low side of the range (around 7.2) while doing this can help prevent stains from returning. If they do DON'T add more ascorbic acid. Just add another bottle of sequesterant and, if needed, drop the pH. The water might turn cloudy for a day or three but it will clear and the stains should lift.
It's really easier to do than to explain so don't let the lengtly explanation scare you! The only problem will come about if you overdosed on ascorbic acid (which is a common mistake). Then it can take a while for the chlorine to hold and the pool might turn green in the meantime, making it more difficult since shocking can bring the staining back. IF that happens then pour your shock amount of chlorine slowly into the skimmer with pump running so if staining does occur it will be more likely to occur on the filter medium that on your pool. If your filter does stain then degreasing and then acid washing it will remove the stain from the filter.
