Ok, I started off with brown/black stains/algae in the bottom of my vinyl liner. I initially rubbed a chlorine tab across the stains and no effect. I rubbed a vitamin c tab across the stains and it seemed to remove part of the stain. So, I decided it was a metal stain and brought FC to 0 and PH to 7.2. Then treated with 5 lbs of AA (25,000 gallon pool). The result was that none of the stain was removed.
I thought maybe my vitamin c tablet test somehow did not give me an accurate result and I then changed my thinking to be that maybe it was a copper stain. The reason being that my gas heater started the season off with a cracked heat exchanger so I decided it must have leaked some copper into the water.
So at that point I decided I would at least get my pool back to normal levels and worry about the stains later. So I shocked it and added chlorine tabs to the skimmer baskets. I did this in the evening and by the next morning my stains were completely gone! Now, I have changed my thinking back to being some sort of organic covering on the bottom of my pool. Today is 1.5 days after shocking the pool and now I see part of the stain/algae returning (maybe 10-25% of the original stain has come back in the exact same spots).
Additional info is that I have never been able to rub any of the stain off of the liner. I have tried brushing and scratching it with a fingernail. I cannot get any to come off on a fingernail so I am still very reluctant to think it is algae.
Any idea what my issue is?