I took a few pictures of some ring stains that have formed on some untouched pools. Before attempts at cleaning the pool, it the ring that formed by the water (in a semi covered environment) was a deep red (above water) with green under tones at water level. After scrubbing and using some recommended stain removers, a pinkish stain remains. It seems as resilient as the greenish yellow ring that remains on our above ground pool. How do we get rid of this, or is it permanent?