http://www.chemistrystore.com/Ascorbic_Acid.htm

The difference between food grade and not is that when they get it in the box form from the manufacturer it is food grade. Once they re-package it in smaller quantities, it can't be sold as food grade. I found this out because last summer they ran out of the non-foodgrade containers, and he called me to ask if I would take the bigger box.

It is the exact same product as the pool store "Ascorbix" and the like and much cheaper. However, Acorbic acid does not fix your problem, it only helps you return the stain to "a solution". A sequestering agent, which you will have to buy at the Pool Store will "hold it there".

Read the Pool Docs Thread regarding Metals and Staining. All pools are different. And I have not found one pool store or pool maintenance company here in Southeastern Mass. that even remotely understands the concepts of this Forum.

I have a 35,000 gallon vinyl lined pool with a heater.My metal comes from my fill water, which is a well. The fill water has a very low PH {5.5}and High Mangenese. I have a very hard time maintaining the PH at 7.0 in my pool.
I chlorinate my pool with nothing but sodium hydro chloride, using a liquimate feeder in a 5 gallon gerry can, and 1/2 to 1 gallon of 12.5 per night.
I never allow my stabilizer level to go over 40.
Filter is 6 hours on, 6 hours off. The liquimate adds just enough SHC to keep me chlorinated . This formula keeps me stain free until the squestering agent wears off, or my ph rises to 7.4, at which point the metal falls of solution and stains my liner. Then I add the acorbic acid, it magically disappears, and the circle remains unbroken.

Good luck to pool owners with wells.