Hi. I have a stand-alone, in-ground spa which is about 6k gallons. I am draining it for the first time since I purchased the house (about 2 years ago). A few months back I had a mild algae problem which was easily treated with shocking the spa, however some minimal staining has taken place on the spa plaster. I do not want to acid wash the spa since it is not that bad. Any alternatives to removing the stains other than acid washing? Would appreciate any feedback.