Well, the Vitamin C confirms it - iron. I suspect over time your well's iron content level slowly increased the iron level enough to cause the staining. Some things to keep in mind:
- Iron generally doesn't leave the water unless you exchange water with fresh water. However some people have had success using polyfill in their skimmer to try and remover iron. That is usually best done when the iron is precipitating out of the water solution.
- You could use a sequestrant to keep the iron in soluble form, but that's a band-aid fix.
- Watch your pH and try to keep it under 7.6 or so. An elevated pH tends to aggravate iron.
- An elevated FC level can also aggravate iron, but you also have to keep algae away.
- If the staining is just in a few places, you can continue spot treatments. If the staining is wide-spread, you may want to consider an Ascorbic Acid treatment.
Ideally, if water isn't too expensive, and/or you get a lot of rain, I would try to exchange some water .... carefully and over time since you have a FG pool and can't just lower the water too much at one time.