I've owned the pool (and house) for almost 6 years. When I first moved in the pool plaster was white with no stains. After about a year I switched from chlorine tablets to liquid chlorine (bought at Wal Mart.) Slowly but steadily I've developed brown stains that I tried using the ascorbic acid to lift but no luck. I tried rubbing a chlorine puck on the stain but it doesn't lift it. The pool was 10 years old when I moved in the house so I don't think its coming in with the water or it would have already been stained. Could the liquid chlorine be causing the staining?