The undergravel filter is perhaps the most misunderstood filter system available for the home aquarium. It has changed little from its first introduction, but this is because of its straightforward method of operation, not from a lack of improvements. At the core of the undergravel filter is a plastic plate that is placed over the bottom of the aquarium, before any substrate is added. This plate is designed to create an open space underneath the plate to allow water movement. Some designs will use a flat plate with different sized slots on the top and slotted attachment points for the “lift tubes”. A more rugged design, made from thicker plastic, has an undulating filter plate with variable slots cut into it. The lift tubes fit into a small socket or well area, providing a more rugged and secure attachment. The lift tube will usually be a 1″ ID clear plastic tube that will rise up to near the surface of the water. If needed, this lift tube can be cut to match the height of the aquarium.

At the top of the lift tube, the hobbyist has two choices, depending on the method of providing water movement, air pump(s) or powerhead(s). If the hobbyist uses an air pump, there will be a flow control exhaust spout placed on top of the lift tube. This devise will direct the water at 90 degrees across the top of the water surface. It will also provide either an attachment for the airline tubing at the top, or have a small hole to slide rigid airline tubing through. Extending down the lift tube will be a section of rigid airline tubing, ending an inch or so from the bottom. Attached to this will be a small, slim-line air stone. Once this is attached to an air pump, hundreds of small bubbles will be forced out of the airstone. These bubbles will rise up the lift tube. As they rise, they “push” water up the column inside the lift tube, spilling with the water out of the flow spout. While it may seem like this would provide little water movement, with the proper air pump, the flow rate can be between 200-400 GPH for each lift tube! The air stone will require replacement every 4 months or so to maintain the proper flow rate.

The alternative to using air pumps and air stones is to use a powerhead. The powerhead will be submersed in the aquarium and placed directly on top of the lift tube. There will be no flow spout, rigid airline or air stone. The hobbyist needs to use a powerhead rated to provide between 60-80 GPH per square foot of undergravel filter. (I.e., a 20-gallon tank, with a base measuring 12 x 24″ would require a powerhead rated at 120-160 GPH.) To replace the water that is forced out of the lift tube, more water must pass through the gravel and under the filter plate. With the undergravel filter, the substrate is the actual filtering material! Ideally there should be a 2 to 3″ depth of medium to small size pebbles/gravel. While the gravel will accidentally become a mechanical filtration system, the real purpose of using an undergravel filter is to use the gravel as surface area for the nitrifying bacteria of the biological filtration system. The constant flow of waste loaded water through the gravel provides the nitrifying bacteria with their preferred food source, either ammonia or nitrite. They convert the wastes to nitrate, a much less toxic chemical to the fish.

As mentioned, the gravel will act as a mechanical media also, trapping the particulate matter from the water. While this can help to provide crystal clear water, there is a danger that the substrate might become clogged and not allow the passage of water through the substrate. To prevent this, the hobbyist must use a “gravel vacuum” (Python system, Gravel Vac or Ultimate Gravel Vac). This will have a large bore tube, 2-3″ ID, 10-30″ long attached to a siphon hose, which might be attached directly to the sink. Once a siphon is started and is removing water from the aquarium, the open end of the large bore tube is inserted into the gravel. As the water passes up this tube, the gravel will gently tumble in the tube and all the trapped detritus will flow out with the water. It is a good idea to gravel vacuum about one-third of the substrate with each (bi-weekly) partial water change. This will prevent clogging of the substrate and prevent accidentally cleaning “too much” of the undergravel filter system.

Since the undergravel filter draws in water over its entire surface, it is important to not physically block more than one-third of the bottom of the aquarium with larger rocks or ornaments. For this reason, undergravel filters are not a good idea in a reef style aquarium. The undergravel filter can also be problematic if used in a freshwater planted aquarium. The large mass of roots can prevent using a gravel vacuum and this will cause the entire system to clog. One option is to use the undergravel filter as a reverse-flow undergravel. This can be accomplished with the use of reverse-flow powerheads or even a specialized set-up with a canister filter. In this second case, the outlet of the canister filter is directed down one of the lift tubes. It is important to have either the powerhead or the canister filter to act as a first stage mechanical filtration system to prevent forcing detritus into the gravel from the bottom. This way the massive surface area of the gravel can still be utilized for biological filtration.

One other consideration is the size of the gravel. Since the filter plates have slots, it is important to use a gravel size larger than the slots. The area under the filter plate must remain clear to ensure proper water movement through the gravel base. Conversely, too large of a pebble size will allow the water to freely pass through the substrate layer and little mechanical filtration will take place. There will also be significantly less surface area for the nitrifying bacteria to colonize.

One last factor to consider in using an undergravel filter is the types of fish to be kept in the aquarium. Since the substrate bed is the key to a successful undergravel filter, if the hobbyist plans on housing larger fish that like to dig and rearrange the décor, i.e. large South American Cichlids, then it is probably not a good idea to use undergravel filters. When the fish dig, they may dig all the way down to the actual filter plate, and the water will now flow through this area with no gravel, disrupting the even flow through the rest of the substrate. These types of fish can also be rather disruptive to the lift tube themselves.

A properly maintained undergravel filter can work as an excellent biological filtration system. Indeed, it was this fact that made it (and the aquarium) a success when it was first introduced. While still a viable option, there have been new methods of bio-filtration developed that might warrant consideration when setting up a new aquarium. These would include the wet/dry filter, the fluidized filter and the Bio-Wheel system. Each of these has their advantages and disadvantages, so it is best to pick the one system that will be properly maintained by the hobbyist.

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