Lenny, fresh city water contains metals which stain the pool. As the water evaporates leaving behind these metals and more water is added, over time the metals become so concentrated that they precipitate on the pool surface in chlorinated water. The way to fix it is unbalance the ph by treating the water with ascorbic acid until the metal is resuspended in the water. But you’re not done there – the pool looks clean, but the metals are still in the water. Therefore you have to drain the metallic water about halfway or so, then refill with fresh water. That should be enough to dilute the remaining metallic water and keep them from precipitating for a while.