Hi there. At the beginning of this thread you reported cya of 80...then cya of 35. Are you confident it's 35, and that the 80 was a misread? Just checking, because cya is neither created nor destroyed...except by addition or splash out!
So, if you have some vitamin c in the house, can you mush some up in a sock and hold it against the stain and tell us what happens?
Chem geek might be the guy to know, but I am wondering if when you supershock to four times your shock value, you aren't just kind of oxidizing the stain (if for example it actually was iron, let's say) into a particulate state than then resettled. Don't know if this is chemically conceivable, but something fishy is going on with this stain by the sounds of it.
What level do you normally keep chlorine at? Do you ever let it fall? How often do you have to lower your ph? What is your water source? How old is your vinyl? You're in ground, right? Have you always used bleach, or previously pucks? Are there any signs of corrosion around your light,
steps, etc.?
My only concern is that if it's metal, you might be exacerbating the situation with such high
shocking. I am personally working on my own layman's theory about these things as I get
confirmed iron streaks and spots too.
Which reminds me, do you fully dose with sequestrate and then sustain dosage weekly? Hopefully
we can help you figure this out
Cheers!