My fiberglass pool just started having a similar coloration on the water line. Followed this thread and crushed up some vitamin c and placed in a sock. Rubbed along the waterline and the coloration faded. I assume it is iron staining?
I am on well water, but have not added any well water within all of 2021 due to the rainfall this year. Unsure why it popped up now. Maybe it has been there and just now noticing more so since my water level dropping to previous levels. I’ve kept a couple of inches higher this summer thinking the natural rain water would be a positive in regards of not adding well water and increasing my TA and CH totals.
That's always tough to know for sure. But at some point the amount of iron accumulated since the original fill does not like the elevated FC, pH, or both. If the staining is not widespread, you can just continue with Vitamin C or Citric Acid in a sock touch-up treatments. If you feel the staining is really bad throughout, then an Ascorbic Acid treatment is in order. Best when your water temps drop just before closing or immediately at opening while the water is still cold to prevent algae. That will (should) remove the staining, but only place the iron into the water. Then you'll still need to manage the water or have it replaced with non-iron water.
When you say “manage the water” does that mean using an sequestering agent to keep the iron soluble in the water and not attaching to the pool? When does the filtering of pool water into with poly fill come into play?
Ideally when the iron has begun to precipitate outwards from soluble form in the water. But that doesn't stop you from trying at anytime you wish just to see if you can manage to capture some particles. It's ideal to remove it via filtering so that you aren't dependent on a chemical later.
Here is a view of where the vitamin c was applied. The first step is an AA treatment? Then followed up with a sequestration agent? Since my pool gets closed for winter I assume it would be best to do closer to the closing since FC should be lowered? Or do at an early spring opening before the water heats up too much in the spring?
Ironically, my PB texted last night about showing some potential clients our pool. He recommended Sparkle Conquest Sequestering Agent. Lastly, I assume adding an sequestered agent without an AA treatment first is a waste of time and money?
That Vitamin C really did it's thing. You are correct about the water temp and timing of it all. The cooler the water the better so you don't have to worry so much about algae. Some sequestrants boast or removing stains and keeping it in solution. Afraid I don't have a special recommendation about that product.