Waste-Away Time Release Gels feature our Waste-Away Bacteria, released into your aquarium at a consistent rate which consume nutrients and break down fish waste, keeping your tank clean and water clear. These gels are REEF SAFE and able to access hard-to-reach areas. To use: (1) Unzip wrapper but DO NOT REMOVE until ready to use. (2) Remove tab, attach suction cup. (3) Stick in area with least water circulation. Cleaning tank with Waste-Away Liquid before using gels is recommended. Available in Freshwater/Marine, single/double pack. Three sizes: Small (up to 10 gal), Medium (up to 50 gal), Large (up to 100 gal). Gels last up to 30 days and change to a tannish color when they need to be replaced.