There are many types of filtration systems that are available to the home aquarium enthusiast. All will work by chemical, mechanical or biological processes. In fact, most will combine two or more of these elements. One such type of filtration system is the aquarium undergravel filter. Also known simply as a "UG filter," it combines mechanical and biological processes to maintain the water in your tank clean and well-balanced.
An aquarium undergravel filter is simple in its design. It consists of a slotted tray, normally constructed of plastic, which is placed at the bottom of the aquarium. This tray design keeps the gravel substrate from touching the actual tank bottom. The tray will also have multiple lift tubes which rise from it. These lift tubes will use the circulatory power of an air pump or power head to lift water from the bottom of the tank and return it via their upper openings. Since the water located at the bottom of the tank has been forced to pass through the gravel by the circulatory forces created by the lift tubes, the water returned from the top of the lift tubes has undergone filtration.
By using this design, an aquarium undergravel filter is able to accomplish the task of mechanical filtration by using the gravel as the primary mechanism for trapping large particles from the water. The system also allows for beneficial bacteria to colonize the depths of the substrate material. These bacteria in turn help to complete the nitrogen cycle, keeping levels of ammonia in check and maintaining the chemical composition of the water in balance.
It is this simplicity of design that also makes the aquarium undergravel filter low in maintenance. Unlike other types of filtration systems which require changing filter media, activated carbon, etc.; UG filters only require that the substrate be vacuumed on a weekly schedule. The vacuuming is accomplished by using an aquarium siphon pump to make a pass over the substrate area to remove physical debris which has been trapped in the gravel by the circulatory forces. Most aquarium enthusiasts combine this action with their weekly quarter-tank water changes in order to consolidate tank maintenance.
Of course, those considering an aquarium undergravel filter should also be aware of some caveats which could avert their effectiveness. The entire success of a UG filter rests with water properly circulating through the gravel and being fed back to the tank via the lift tubes. If debris is allowed to accumulate in excess within the substrate, this will compromise water circulation and the continued effectiveness of the mechanical filtration effect. This in turn, will also lower the effectiveness of the beneficial bacteria in completing the nitrogen cycle, allowing levels of ammonia and nitrites to rise to dangerous levels quite rapidly. As such, adhering to a routine maintenance schedule is critical in order to maintain the high effectiveness of an aquarium undergravel filter.
If one intends to have rooted live plants in the aquarium, one should also be aware that aquarium undergravel filters can impede their growth. In order to make them compatible with one another, when installing the bottom plate of the UG filter, place barrier plates over the grates that will rest immediately below the area that you plan to use for anchoring rooted plants. In doing this, do not cover more than 15% of the surface area of the grated bottom plate so as not to have circulation problems.
An aquarium undergravel filter can be very effective and is one of the most popular filtration systems used by aquarium enthusiasts for decades. Provided that you adhere to routine maintenance schedules and be vigilant for the signs of reduced water circulation, they can serve your aquarium well.