A thousand thanks, Marie. I'll try to do the test today, but I need to do it when it's not sunny and, as you know, it's a beautiful day here in the tri-state area. So may have to wait a bit. Pool closes in 2 weeks, so I need to get it done! Hard to believe the the "gel" from a Vitamin C tab will remain in a sock, but I'll give it a try!

Do you have a San Juan pool ? I was wondering because the 16x33 is sort of uncommon (at least the 33 is) and we both have the same size. Mine's an Atlantic.

Anyone, I've been reading the manufacturer site and it says don't use a sequester (not even sure what that is). They recommend Jack's or Metal Magic. See: http://www.sanjuanpools.com/maintenance.html

Reading about Jack's, it says it's "heavier" than water, so I guess it floats down to the stain.

In any event, looks like it's treatable, which is great news.

Again, thanks!