Our house is fed by a spring. The water is soft and so on the acidic side. We had a new upstairs WC and sink fitted. However, over time the WC has gained some green stains down the inside back, where the water flushes down. I am guessing these are caused by the soft water and maybe some interaction with the copper piping. However, nothing seems to remove them - they appear almost to be under the glaze. We are also on a septic tank so need to be careful with cleaning substances. There have been no similar problems with the wash basin. Any ideas on how to shift them and how to prevent them coming back?