I have a 24' above ground pool & have patchy brownish spots on the bottom of my pool. I was told it was metal & to use Ascorbic acid. I did & followed the directions & they're still there. Next I shocked it to see if was aligy & it's still there. In had my pool water tested @ Leslie's Pool & it tested fine, no metals but they only checked for copper & iron. What is this & how do I get rid of it?