The Water Softener Pillow removes hard water minerals and toxic heavy metals from aquarium water. It consists of a special "pillow" that fits into the RENA Filstar Canister Filter, providing continuous removal of minerals and heavy metals. Providing an aquarium with softer water improves fish health and can prevent unsightly mineral deposits from forming on the aquarium surfaces. Many fishes, such as tetras, angelfish and South American cichlids prefer the soft water common to the Amazon River Basin, (50 to 100 pm GH, typically). The Water Softener Pillow helps to re-create the soft water found in many tropical areas of the world where aquarium fish come from. Maintaining a stable aquarium environment is much easier when using the Water Softener Pillow.