Here's the thing I'm wondering about...
You've done partial water changes and I think even a total water change. Normally, dilution would reduce the ppm of any iron or copper level and thus reduce the volume of sequestrant needed subsequently to keep whatever's left in suspension. In your case, the sense is that dilution isn't making a difference or impacting your volume of sequestrant consumption...correct?
Dilution of concentration is how it worked for me, but only after I'd both switched to softened water on the outdoor spigot and started using a 10" Pentak filter with a 25 ppm to 1 ppm filter on the spigot.
At the same time, if you don't have a home water softener (do you?) and you had that much iron in your source water, you would see iron staining in our dishwasher, toilet etc. If you don't have a softener and you don't have iron staining, then your source water is not likely the culprit.
Salt can in fact introduce low levels of iron to the water...each time you've refilled, have you used the same brand of salt?
Right now it sounds like you're entirely bypassing the heater; we know your cell is stained; and we also know you've checked every nook and cranny for eroding metal.
If all the above are true, ie, you've had heater bypassed entirely since last dilution, you added a different brand of salt, your home fixtures aren't stained, etc. that makes me a bit suspicious that something is happening either below the liner or something not bonded is exacerbating what would otherwise be mild iron issues in swg due to trace amounts of iron in some brands of salt etc., or in your case, soil blow in and rain with trace amounts of iron.
Iron bacteria in wells comes from iron bacteria in the soil and is usually addressed by chlorine injection, but at a much greater concentration than in a pool. While that fact in itself isn't helpful, what happens if you slam (have you ever?) - does the metal tint the water, do the stains intensify?
Am I correct that your liner is 5 years old but you've only had significant staining problems the last 3 years?
How old is your pool? Is the underground plumbing PVC for sure? Not copper?
When you get your Iron test kit, test the water when the stains are back - and your outdoor spigot water. If you show little or no iron etc. my offer stands to buy the tester from you, btw
Another strange question is do you use borates?
It might have been a different thread, but Joyful Noise pointed out recently that borates can reduce the internal ph in a cell, I believe.
Boric acid also came to mind because I was thinking about those wood walls under there...I rehabbed my forclosure crawlspace wood with a boric acid treatment that in effect sterilized it against all growths. If its technically possible for iron bacteria to grow on wood, I'm wondering if a borate perimeter treatment is viable.
I've never heard of such a thing, BUT I have run across rare cases of under-liner staining where a perimeter treatment of ferric sulphate (which sounds crazy, as yes, ferric means iron) has eradicated under liner issues by changing the ph of the soil.
Others report using fungicides, which boric acid it.
Here's a link to an article about under liner staining:
Vinyl Liners: Tackling the Toughest Stains of All, the Kind Behind the Liner| Pool Spa News | Maintenance, Quarterly Market Reports
