Lees Premium Under Gravel Filter for AquariumsLees Premium Under Gravel Filters clean your freshwater or saltwater aquarium through a biological filtering process that supports the growth of beneficial bacteria. This allows natural detoxification and maintains proper ammonia balance by circulating water flow through the gravel bed. Under Gravel Filter Circulates Water Flow Through the Gravel Bed Requires the use of an Air Pump not included Maximize Water Quality by Using a Power Filter Provides