Honestly, if I'm being charitable and kind in my technical opinion of Periodic Products, I believe most of what they sell is junk and about one degree removed from snake-oil....
And as far as iron-bacteria goes, it is
exceedingly rare to have any appreciable growth of iron bacteria in pool water unless the pool is a complete green swamp. Iron-bacteria is highly susceptible to chlorine sanitation; in fact, chlorine is injected into water wells to kill iron bacteria. Iron bacteria is found mostly in well water because deep wells have almost no dissolved oxygen in it and, because the bacteria needs an electron transport system to power cellular life processes, the redox reaction between Fe
2+/Fe
3+ is used. It's an evolutionary adaptation that occurs when bacteria live in extreme environments similar to how bacteria that colonize deep ocean geothermal vents live without any sunlight or oxygen by exploiting the redox reaction of sulfur and hydrogen sulfide. So the notion that anyone would have iron bacteria in a clear swimming pool with a detectable level of chlorine in it is laughable.
As for the CuLator stuff, well, until I see actual proof written up in a peer-reviewed journal or tested independently by a scientific lab, the concept is also highly suspect. It's basically a bag of ion exchange resin that you throw into your skimmer or pump basket. So the expectation is that this ion exchange resin is going to "absorb" all the metals in your pool water?? How exactly? Most of the water flowing through the plumbing
will flow around the bag, not through it. So what...are they counting on diffusion of the metal ions into the resin?? Natural diffusion processes are extremely slow and, once the surface of the resin is fully loaded, diffusion into the bulk of the resin bag will be even slower. So how is this thing actually supposed to absorb anything from the water when 99% of the water flows around it at high velocity?? It simply makes no sense from an engineering or hydraulics perspective. That too coupled with the fact that most ion exchange resins are NOT metal specific...any divalent metal (Cu, Ni, Fe....calcium, magnesium, etc) will be captured by the ion exchange resin. The calcium levels in pool water are several hundred times higher than any metal concentration, so how exactly does this resin bag get loaded up with only one specific metal ion??
I'm not trying to make anyone feel bad, but these products are akin to snake oil at best. And, as Pat said, they just sold you an algaecide that adds copper to the water and now you have to get it out somehow (and I don't think the CuLator is going to help).
My best guess is (and I haven't read all 61 posts) that you simply have a lot of dissolved iron in your water and possible tannins from all of the pine needles. You live in Georgia which is an area of the country notorious for well water that has lots of iron it from the heavy clay soils that are rich in iron compounds as well as the larges amounts of decaying organic matter that acidifies soil and drives the iron into the well water and surface water. The stains keep coming back because as the chlorine and pH increase in your pool water, the iron is simply not stable and comes out of solution. Why does it always show up at the same spot, well, that's actually fairly common. Unlike a plaster surface which tends to develop scale, metal does not like to scale out onto vinyl surfaces. So there is something about those areas of the vinyl that allow for the scaling of metal to happen more quickly and easily than other areas. Typically surface roughness will enhance scale formation and so the vinyl may be slightly more rough in those areas (imperceptibly so to hands or eyes) and so those are the first areas where scale will form. As
@AndyTN showed, PVC surfaces quickly become brown with iron because they are fairly rough on a microscopic scale.
If you cannot fill your pool with metal free water, then you have to either try to precipitate as much of it as you can in your filter or remove it with hose-end filters on the fill line. You will need to keep your pH on the low side (7.2-7.4) and use sequestrants to try to hold the iron in solution. Short of having an RO filtration company come out and filter your entire pool volume, metal scale is not going to be easily solved. I do feel bad for you as it is an annoying problem to have but, luckily, iron scale is just an aesthetic issue and not a sanitation problem.
Good luck to you.