I've had my inground vinyl lined pool for 7 years now. I've been at the mercy of the local pool supply place in my area. they are very expensive and I have decided to start learning for myself how to take care of my pool. I have a salt water chlorinator and sand filter. I did not ope my pool last year due to the expense of opening it. I left it covered and winterized. Upon opening it this year, I found an absolute mess! I have finally gotten the water some what clear and blue but have found that the lowest 2-3 feet on the walls is a dark greenish black. I just assumed it was algae that needed brushing off. It does not brush off. I'm not sure what it is. I'm wondering if I developed a copper problem while the pool was closed over the last year and a half and now have stains from it. Could someone please shed some light on the situation. I know if I go to the pool place they'll end up selling me $500 plus of various chemicals to put in it with the promise of clearing the problem up. I know there has to be another way more cost effective. I appreciate any help I can get!!!