We have an inground pool with a vinyl liner. It's been nice and clear for years. About a year ago we installed a Hayward heater. 6 Months later the heater core started leaking and the copper in it had disolved due to low PH Hayward told us (they were quite good to deal with by the way). This year for the first time, we've had grayish/black stains appear more and more frequently randomly around the pool. Out plastic skimmer and bottom drain have also turned dark for the first time. The pool water is superb however - no cloudiness and perfectly balanced now. Shocking the pool with high chorine level seems to help, but they come back quickly. We got a stain eraser and it has no effect at all on the stains. I suspect we have copper in our water from the heater core portion that disolved which may be the root of the problem. Any idea how to get rid of these stains...?