Because it's easy with my SWG, I've kept my FC rather high all summer (7-9FC with 60-70CYA), figuring it would give me a buffer for high bather usage, or if there's a problem with the SWG while I'm away on business trips.
I'm noticing metal staining which I suspect is due to the high FC paired with the occasional rise in pH over 8 when I'm not home to deal with it (new plaster this year). Well, that the metal staining is showing because of it...not sure where the metal is coming from (I've got an old gas heater, and the water did turn clear green when they filled after the replaster; I didn't realize I needed to keep adding sequestrant after that initial dosage the plaster contractor added. I've confirmed that the stains I have are metal via vitamin C tablets, and have bought a couple bottles of Jack's Purple stuff...with my FC at 7, what would happen if I just added it now instead of trying to lower FC first? I'm only home for this week, and then I leave again for 2-3 weeks, so I definitely don't want to try to lower FC and then leave the pool in a position to let algae set in with no one watching it close. Even now with lower temps, we get a LOT of leaves, and unless I'm home they will sit for a day or two before my lovely wife will get them out, further scaring me from dropping the FC.
While I understand that there is a potential for reduced effectiveness of the product at higher and near-zero FC levels, what exactly occurs? Does it reduce the FC (thus lowering FC instead of sequestering metal), or does the FC kill off the sequestrant entirely in some way? Will tossing it in now be a complete waste of money, or might there be some benefit to it, though less than optimal? If there is benefit (though reduced), I'm inclined to toss it in now and continue the effort when I return, but if not, I'm thinking I'm better off just waiting a few more weeks.