IceCap PRO Internal Turf Scrubbers use algae as a natural water filter to remove nitrate, phosphate, and other pollutants from your aquarium water within the safe confines of your sump. Rather than having algae grow uncontrolled inside your display tank, an algae scrubber allows you to grow algae in a controlled setting where it can be easily removed—thereby removing contaminants from your tank. This all-natural form of water filtration can reduce and even eliminate the need for water changes.