Penn-Plax Hang on Power Filter. 
Quad filtration system:
Mechanical- Polyfiber Floss Cartridge, traps floating particulate matter.
Chemical- Activated Carbon Cartridge, removes harmful chemicals, toxins, odors, discoloration and other contaminants.  Internal shelf system spaces carbon throughout cartridge. Prevents teabagging.
Biological- Internal Sponge,optimizes colonization of benefical aerobic bacteria. Removes ammonia and nitrites.
Biological-External Bio-Falls, wet/dry biological booster for additional ammonia and nitrite removal. Enhances oxygen exchange for maximum water oxygentation.
Other features include: 185 Gallons/hour, up to a 55 gallon tank.                  Adjustable flow control, Motor-quiet, efficient and maintenance free, Extendable lift tube for maximum depth of tank, Leveling device-keeps filter positioned and stable in most aquariums, Easy disposable replacement cartridge-free cartridge included. Replacement cartridges sold in single and triple packs for your convenience. Also available in four other sizes.