I was replying while you were typing It does sound like you may be getting the metals through the air. It does sound like iron. I think you will have to use a lot of sequestering agent - putting some in every week to maintain what is lost through backwashing and dilution. Is there a municipal water testing place you can take your water to? It can give you a more detailed list of what metals are in the water. I wouldn't trust a pool store that said you had a copper level of 5 and didn't say anything to you. You can live with a little staining, but a copper level of 5 would not be advised. If you are going to get one more year out of your liner, then just use the sequestering agent, and keep up with the stain with a little ascorbic acid every now and then. Just be sure to check out the copper. You also must keep an eye on the ph - ascorbic acid will lower the ph too.