Several years ago when having problems with stains, probably iron, I got rid of the stains using ascorbic acid (I forget the exact procedure) and started using Jack's Magic Purple Stuff to keep the stains from returning (initial dose of 2 qts, then 12 oz each week in my 20k gal vinyl IG pool). I've been using it for several years now and they stains have not returned.

Speaking of using HEDP sequestrants, I understand that chlorine breaks them down. Ben, does that mean that my regular addition of HEDP (I add 12oz of Purple Stuff each week per instructions) is increasing chlorine demand? I wondering if by using this product I'm shortening the life of my salt cell?

Something else I don't understand. I know the HEDP sequestrant keeps the metals in solution, but you mentioned that as it breaks down the metals will end up in the filter? The Purple Stuff I use claims it allows the filter to trap the metals so they won't stain the pool. Does this mean that is some indirect way the HEDP helps remove the metals via the filter?

BTW, I'm in the process of testing my pool for metals using your bucket technique you suggested. I'll let you know the results.

Thanks.