You can try rubbing some Vitamin C tablets crushed in a sock or stocking on the stain, or if you have some ph down you can use that instead - if it lightens up it is a metal stain if it stays the same it is probably organic. They are telling you to put a sequestering agent ("Suspend M+S") to bind with any metals that are in the water. You won't need this if the stain isn't from metals.