If it's metal stain- using an orange, lemon, or lime rub an area with the cut half; if the stain starts to disappear after about 20sec. you can buy ascorbic acid at a local pool supply store. You'll also want to use "Metal Out" to prevent the stain from coming back. Also check your chlorine level, high levels produce the high metal content.
Faint brown stain on pool bottom
in Swimming Pools in France
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