There are two links in the post you copied that are a cheaper way to get ascorbic acid - the second website you can get 5lbs for $45.
http://www.msm-msm.com/store/agora.c...scorbic%20Acid you will need about 3lbs. I use a pound for every 10,000 gal. You can go around the perimeter of the pool, it doesn't have to be exact - that's why you put the pump on circulate for about an hour, so that it mixes well, and will take the all the stains off. Don't forget that once all of the stains are gone, you have to put in the sequestering agent, so that the metals can bind with it, so that they dont' fall out again when you start to add chlorine. Let me know if you have any questions. Don't feel like you should have known about the cya chlorine thing - the pool stores don't ever tell you that - and half of them don't even know it themselves Stick with this forum and you will be able to have a clean clear pool all summer by taking care of it yourself