How do you get stains out of plaster?

My 20000 gallon gunite pool is about 3 weeks old now. Part of the initial fill was with well water (high in iron - therefore I'm assuming stains are iron related). I ran the garden hose all night before water tanker arrived in morning and filled with 18K gallons. I brush pool twice a day.
Now I am noticing some light brown or tan colored stains near the main drain and swimout. They do not brush out. What should I do next? The water is balanced - I have Ben's test kit.
My readings are:
FC = 2
ALK = 90
CAL= 160
CYA = 40
PH = 7.5
TEMP = 78

Check if the stain comes off with vitamin C (you can put a couple of them in a sock and rub it over the stain) If it comes off then you will be able to get rid of it with a stain treatment that has ascorbic acid. But if they are not bad, I would use something like metal free first, let your filter run 24/7 for a while. Sometimes the stains will come off, even though metal free doesn't take stains off pool surfaces, it sometimes works if the stains have not been there long. I can't answer why, but I have seen it happen. If the stain remains, you can go for the whole stain treatment. Let me know if you have any more questions, and I will be happy to help.

Do I buy a bottle of Vitamin C from the grocery store (for human consumption) or are you talking about something sold at pool stores?

You can just use vitamin C you get at the drug store. If you need to do a stain treatment you can buy ascorbic acid (that is what vitamin C is) in bulk - here is a link:

http://www.msm-msm.com/store/agora.c...scorbic%20Acid

Let me know if you have any more questions, I am glad to help.