Natural Chemistry’s STAINfree is an ascorbic acid that removes metal stains from pool liners and finishes. After balancing the pool’s water and lowering the chlorine level to 1ppm, you can poor the STAINfree in with the pump off overnight. After allowing it to soak, you will brush the walls before turning the pump on again. You will not want to shock the pool for at least a week. The treatment should help with any surface staining or water discolouration.