I had my pol re plastered in May 2013. Two weeks later stains began to form on the bottom of the pool. Contractor blamed the chemistry, PH was high. The ph was balanced but the stains coninued to show up and spread, only on the botom. Check for metals, no metals. Used Jacks Stain removal, nothing, used asobaic acid, nothing. Steel brush and sanded, still there. What's uo?