New to the forum - Great information - I wish I had discovered this years ago!
Had a problem with stains on the vinyl liner and steps last year. Removed by treating worst areas with oxalic acid in a sock and a treatment with Metal Magic. Stains re-appeared this year on opening the pool (in-retrospect after adding baking soda and shocking the pool). Disappeared with a vitamin c tablet. Water tested no metals but added 4 bottles of Metal Magic per pool store recommendations. Followed direction on the bottle to the letter. Stains disappeared but filter clogging daily. Chlorine levels low (0.0 - 0.5 with strips) but waited the two weeks before shocking or adding much chlorine (was using 3" tablets then). On adding baking soda to get pH up over 7.0 the stains re-appeared. Still backwashing the filter every few days. Back to pool store and they said to add another 4 bottles of Metal Magic. Added and the stains again disappeared. Water has been crystal clear the whole time. Free Chlorine 0.0 and total chlorine 1.0 (per pool store). Added more tablets. Can't get any free chlorine level and changing the filter daily. Back to pool store and they say phosphorus level is high and that is why chlorine levels are low. Sold my wife something for phosphorus.
At this point I wise up and guess that the phosphorus has nothing to do with the low chlorine, guessing that the Metal Magic is binding the chlorine and that is why I'm going through chlorine and DE like crazy. Went internet searching and discovered this forum and re-affirmed my suspicions that baking soda bleach are better than the pool store bad information. Problem now is that I'm still going through Clorox bottles and DE like crazy. Water looks beautiful. TF test kit numbers - Chlorine levels good after addition but go to 0.5 by the next day. Total chlorine 1.0, pH 7.5, CYA 100, alkalinity 150. Free copper looks like 0 or <0.5. I'm buying so much bleach at Sam's that they must be wondering if I'm cleaning up a crime scene (Truth is I am - ripped off by bad pool story advice over the years.)