Aquarium Undergravel Filter

When an aquarium is not well maintained, waste solids build up in the gravel and can clog media beds, coat plant roots, and ultimately accumulate into toxic anaerobic sludge. A simple, inexpensive aquarium undergravel filter (UGF) can help control this by drawing the gravel down into an open plenum beneath the tank and slowly breaking it down in a constant, gentle water flow. Large designs often incorporate separate appliances to handle excess solids, including techniques that transfer the material into a mineralization tank where it can be stirred and aerated until it breaks down to a nutrient-rich liquid form. But the basic UGF can do this just as well, especially in smaller tanks and for plants that don’t require heavy rooting.

The undergravel filter works by relying on the fact that the surface of each gravel particle is inhabited by aerobic bacteria that metabolize nitrogen-bearing compounds like ammonia into forms that are less toxic to fish. These bacteria are aided in their work by the tiniest of pores on the gravel surface, which provide them with a huge amount of surface area to support large populations of aerobic organisms. These bacteria transform ammonia into nitrite and then into nitrate, which is more readily removed by the aquarium’s regular filtration system.

To operate a basic undergravel filter, the aquarium is filled with freshwater or saltwater of appropriate temperature for the specific tank inhabitants. The bubbler tube and flexible air tubing are rinsed thoroughly with dechlorinated water. After the UGF is plugged into the aquarium’s filter outflow, an upward current of water is created by pumping air through the bubbler tube, which entrains the tank’s water and pumps it upwards. The filtered water then drops back down through the gravel layer and into the tank.

The advantage to using an undergravel filter is that it is cheaper and simpler than many other types of filtration equipment. This makes it an excellent choice for beginners or for a home aquarium that doesn’t need a high-tech filtration system. However, the reliance on the aerobic bacteria that occupy the gravel’s surface can lead to problems with algae growth if the aquarist fails to keep up with regular maintenance.

The undergravel filter should be cleaned once per week, removing the organic waste build-up in a quarter section of the tank at a time. This prevents the bacterial colonies from being wiped out and allows the colony in each section to repopulate before siphoning again. The UGF can also be agitated periodically to help dislodge the accumulated solids and make it easier for a vacuum or siphon to remove them. A simple aquarium vacuum can be used to clean the undergravel filter, but a siphon is more effective. The undergravel filter is also not as efficient in preventing water loss due to evaporation, so an aquarium thermometer is required to monitor the tank’s water level. Regular upkeep and regular water changes will ensure that an undergravel filter provides years of trouble-free filtration.