The Penn Plax Penn Plax Undergravel Filter combines the function of a traditional filter while offering maximum viewing since it’s hidden out of sight. Designed for 40 to 50 gallon fish tanks, this undergravel filter uses mechanical, chemical, and biological filtration to filter harmful pollutants from your aquarium. Undergravel filters create the most natural bacteria bed for fish tanks and provide effective aerobic filtration. Included are 4 filter plates (each measuring 11.5" X 11.5") with extra large 1 inch adjustable lift tubes. These lift tubes carry bubbles from the bottom of the aquarium to the top creating a gentle flow of water through the gravel and up the lift tube. Designed with narrow slat openings, the gravel is prevented from falling through to filter cartridge. Air pump and airline tubing not included.