I've had several pools with various problems but this one has me baffled. Bought home in winter 2015. Above ground pool crystal clear when opened. August dark stains appear so I find TFP website and am very impressed. A vitamin C tablet dissolved it in an instant. Late in the season so I decided to wait until opening this year. Cover removed and stains were COMPLETELY gone. I'm confused but curious. August this year they come back. I let the chlorine drop to a safe level and add 3lbs of ascorbic acid and let in run on recirculate overnight and they're gone again. I add a sequestering agent to the skimmer as well. Now there are very faint stains but the pool is using at least three times the chlorine as before. I don't understand why?? The water was complete balanced before I added the three lbs. of acid. Any ideas? If it were metal, why does it disappear under the winter cover?