not sure. It sat up all winter long and I did circulate it every few days but leaves other debris got in there;. I assumed it was a vegitative stain til I tested it with a couple of Vitamin C tabs. I put them in and where they were sitting, the liner was clean. From what I had read, that would be the vase. The Vitamin C tabs would clean a metal stain. When we bought the house they did not disclose the pool had any problems. NOW I find out about them form the neighbor. He said the other folks thought they had all the problems fixed. NOT