The Lifegard HOF Hang-On Filters come in four sizes. The smallest (HOF-10 for tanks up to 10 gallons) has one filter cartridge, the HOF-20 gallon has two, the HOF-30 has three, and the HOF-40 gallon has four. Ideal for freshwater or saltwater Aquariums. The surface skimmer removes floating debris or oil leftover by feeding dry food. The top control system knob lets you slow down the water output, creating a gentle flow to protect delicate fish and plants. These filters are slim and can be closer to the wall than competing filters. Useful Links: Best Aquarium Filter?