Hi. I have some stains on my vinyl pool liner that have already responded to ascorbic acid treatment 2 years ago but the spots only got lighter, they didn't completely go away and I just stopped trying. I want to try and get rid of them completely and so I was reading the article about ascorbic treatment to rid pool of metal stains (Ascorbic Treatment to rid Pool of metal stains). I see that it says that the chlorine level should be 0 so I'm assuming that when I open my pool that should be the perfect time to do this before adding any chemicals. Am I correct? Also, once the stain is gone it says to add a sequestering agent...what is that? I have never heard of that. Can someone recommend a good brand and suggest where I even buy this? Thanks in advance!