GLB Stain Magnet is a stain remover for use in all types of pools including gunite, plaster, concrete, vinyl, and fiberglass. This easy to apply granular product allows you to remove metallic stains without the inconvenience of draining pool water. It can be applied directly on stubborn stains with no pre-testing necessary. Stain Magnet is compatible with chlorine, bromine, ozone, biguanide, and salt water sanitizing systems.