These are different types of filtration systems. They are ranked by there usefulnes.

Under Gravel Filter

An undergravel filter is simply a slotted piece of plastic (undergravel plate) that sits on the bottom of the aquarium and lets water flow between the plate and the bottom of the tank. There is a tube that is attached to this bottom piece, known as the lift tube. Gravel is placed on top of the filter, and the filter, lift tube and gravel work together to provide filtration. Water is pulled down to the bottom of the tank, through the gravel. And then the undergravel plate, then flows back up to the top through the lift tube. The water is again pulled down through the gravel. This allows beneficial bacteria to thrive on the oxygen laden water flowing through the gravel.

A Power Filter is a chamber filter that sits on the out side of the tank hanging to the inside, the water falls like a water fall. the water is sucked up by a filter tub then pushed threw a filter cartage with carbon and floss, then pushed farther out to it falls out of the top of the filter and into your tank.

Compact filters feature the rotating Bio-Wheel wet/dry biological filter, proven to remove ammonia and nitrite faster. Self-starting pump restarts automatically after shut off. Designed to be leak proof and easy to maintain. Flow rates are certified accurate.

Filters allow you to achieve maximum efficiency in multi-stage (biological, chemical and mechanical) filtration using a variety of filter media. The design of the filter divides the media into easily accessible "baskets" which allow you to adjust their types and proportion as needed to achieve the desired results. Includes all necessary components as well as "starter" portions of activated carbon media, BioMax filter media, designed to host large colonies of beneficial bacteria, and Fluval Foam Blocks to mechanically filter particulate waste.

A complete under-the-tank wet/dry system offering mechanical, chemical and biological filtration with 100% water-to-media contact for superior water quality. Designed for flow rates of 300-500 gph, this heavy-duty high capacity filter features a patented Bio-Wheel that instantly removes nitrite and ammonia, and three slide-out filtration trays housing a filtration pad, 16 oz. of activated carbon and additional media of your choice. Perfect for fresh or marine aquaria.

The patented helix design forces water to make a double-spiral pass along the entire length of the PowerCompact UV lamp. This Helix UV technology provides unprecedented 36" and 60" of Ultra-Violet exposure. That's more than twice what conventional UV sterilizers supply! The result of this exposure is a dose of ultraviolet light that kills over 98% of harmful microorganisms, eliminating unsightly algae and disease in your tank.