The CF-500UVMKII takes UV canister filters to the highest level with new and improved features such as Vortex Prime, Secure-mount Fittings, and an optional Surface Skimmer! This 525 GPH 4+1 filtration system is ideal for fresh and saltwater aquariums up to 175 gallons. The integrated 9-watt UV helps keep your aquarium healthy and crystal clear. The CF-500UVMKII defies its low price because it comes equipped not only with UV but all the necessary plumbing for installation and includes four large filtration media compartments allowing you to add the filtration media of your choice. The filtration media trays offer a fully customizable filtration system to suit your specific aquatic needs. The CF-500UVMKII includes four fine filter pads (white) and a single coarse filter sponge (blue) for enhanced Mechanical Filtration. With the addition of Aquatop's Premium Activated Carbon, Ceramic Rings, or Bio-Balls (included), you'll achieve superior filtration and crystal-clear aquarium water. The CF-500UVMKII Canister Filter has two convenient Priming options that eliminate the need for manual siphoning. Say goodbye to a mouth full of aquarium water. Unfiltered aquarium water enters the CF MKII Series Canister Filter and passes by the U.V. Bulb flowing down the stack of vertical cylinder cutouts to the bottom of the CF MKII Filter. Then water is pushed up through the filtration media compartments, the bottom compartment first, the middle compartment second, and the top compartment third. The filtered aquarium water then returns from the CF MKII Canister back into the aquarium.