I have identified stains on my pool liner are iron by doing the vitamin c in a sock test. It was amazing how the stain lifted immediately when I moved the sock across the stain! I immediately ordered a bucket of ascorbic acid and have it coming to the house. I am following the ascorbic treatment instructions. It mentions a sequestering agent being used before bringing the chlorine up. Upon looking online at some of the recommended sequestering agents, they look expensive. Is there a recommended sequestering agent that is more cost efficient?