Looks like it could be either an organic or metal stain, but we'll need more info to tell (of course

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Can you post some test results (FC, CC, pH, TA, CH, CYA) and how you obtained them? How are you chlorinating? Do you have well water or an ionizer system?
I seriously doubt algae under the liner. Based on people's posts I've seen here, "algae or fungus under the liner" has always turned out to be stains on the surface, caused by either metals in the water or more commonly, insufficient chlorine.
Hi, i tried the vitmin c test and it looks like its a metallic stain. can you please advise as to how get rid of it if its possible thanks