I really don't understand - if a vitamin C tablet worked, then the ascorbic acid should have worked. The low ph and no chlorine should have made this a snap. Are you sure it was ascorbic acid or oxcylic acid? I am really stumped. You can contact the pool company and ask them the best way to drain - they have to drain them when they have to fix cracks on the bottom. I have a friend who had to have this done, I will call her tomorrow and ask her how it was done. You must really be frustrated! I feel your pain. Keep the faith!!