Marie,
I have not added a sequestering agent yet but had planned to after I did the ascorbic acid treatment. I would usually then backwash/replace DE and add more sequestering agent after all is said and done. Maybe I'll try just the sequestering agent first and see what happens. I'll let you know.
Now let me clarify, these stains are not that noticeable in full sun but usually in low light, such as evening, and I notice them more than others (maybe OCD?).
One observation I have made so far is that the stains appear to be occurring where the gel coat was sprayed in intervals. This is the only explanation for the almost perfectly straight lines that divide my pool in to 3 equal parts. I know that when gel coat is sprayed, you can usually only spray up to about a gallon or so at a time due to "kick off " times (the time in which the gel coat will start to cure). So it appears that where one part overlapped the other, staining is more prone. Makes we wonder if maybe there are slight indentions or something in these areas that collect the stains. After all, fiberglass/gel coat/resin is not absorbent so the stains occur at these locations for some reason.
Either way, It really isn't that big of a deal and if I had to do it all over, I'd still choose the same pool, stains and all!