Only at levels in excess of 1ppm or running a prolonged high pH level. The ES3 system operates at 0.25-0.4ppm. If the system is unattended for some time (more than a month) and not set appropriately you might see some staining. However, it can be easily removed with citric acid a commonly used pool stain remover and causes no permanent damage. To put our required copper operating level in perspective, municipal drinking water supplies in Australia are permitted to have 1.2 ppm of copper in their supply to households. It is possible that some non- Enviroswim pools already have copper levels higher than Enviroswim recommends.