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WE have been having some issues in the deep end with black stains, we tried the Vit C but did not really make a difference. It is a fairly new liner about 2-3 years old. I have been reading on this site about FC how would we increase this. What other ideas, sadly we have been at the mercy of pool store with not much improvement.
Pool is 20 by 40, inground
Black stains on liners:
1. Copper oxide -- should be affected by Vitamin C
2. Iron oxide -- less common than red oxide form; should be affected by Iron Out (detergent section of grocery - use a sandwich baggie to place powder directly on stain.
3. Fungal staining -- from some form of fungus living on the DIRT side of the liner. Usually appears where liner touches dirt, not pool walls. Usually occurs when dirt under liner has been water saturated.
#1 and #2 can be lightened, and often, removed. #3 cannot. Direct application of cal hypo may lighten, but not remove it, and it will return. The pigment from the fungus bleeds through the liner, but since it is going THROUGH the liner, there's no way to get to the fungus, without replacing the liner. If you do try cal hypo, do not leave it in contact with the liner for more than 10 - 15 minutes -- it can bleach the liner white in that spot.