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Undergravel Filters: Classic Simple Filtration

Undergravel filters have been around for a very long time, and were commonplace in fish rooms and retail fish stores for many years. With recent technological advances in filtration, this simple, easy filtration method saw reduced popularity as aquascapers started using power filters like canisters, internal filters, and hang-on-backs. The newer filtration systems allowed for the use of delicate aquasoil and fine sand, however they added mechanical complexity and lots of equipment that needed to be hidden, as well as a high price tag. While undergravel filters must be used with gravel, and cannot be used with sand, soil, or other fine substrates, they are cost-effective, reliable, and low maintenance filters that can last the lifetime of your aquarium.

How Do Undergravel Filters Work, and What are the Benefits?

The way undergravel filters work is by using an uplift tube to create flow, much like a sponge filter does, attached to a grid of plates under a couple of inches of standard aquarium gravel. The water is drawn downwards through the gravel, which creates a surface area for beneficial bacteria that extends the full floor of the aquarium. Once the water is drawn through the gravel, it is siphoned upwards by the uplift tubes, and flows back out into the aquarium. Debris, detritus, and fish waste gets trapped in the gravel, leaving water crystal clear and free of toxic ammonia and nitrite. In addition to being incredibly effective, undergravel filters provide a gentle form of filtration that is one of the safest options available for long-finned bettas, tiny fry, and shrimp.

Undergravel filters require very little in terms of maintenance, and can typically be maintained without removing any equipment from the aquarium. Cleaning this filter is as simple as a normal gravel vacuuming procedure, as you would when deep-cleaning any aquarium’s substrate. If you aren’t familiar with how to gravel vacuum an aquarium, you can find a detailed guide here. If your undergravel filter is being used with the air stones that came with it, you shouldn’t ever have to worry about cleaning the air stones, either, as the solid plastic construction resists mineral buildup and clogging. Their low maintenance requirements, customizability, and broad application potential make these undergravel filters an excellent choice for large fish rooms and heavily stocked tanks, in addition to being very easily hidden behind plants and hardscape. Just like sponge filters, this type of filtration runs on air, so you can use a battery backup air pump to have reliable filtration, even through a power outage of up to 40 hours.

Can Undergravel Filters Be Used in Planted Tanks?

It has long been rumored that undergravel filters are not very compatible with planted aquariums, however we have found that not to be the case. As long as the correct type of substrate is being used, plants can thrive in a tank with an undergravel filter, and this filtration method has the added benefit of constantly moving oxygenated, nutrient-rich water through plant roots. This can prevent root rot, increase nutrient accessibility, and create stronger root structures. There is a chance that your big Amazon sword, or dwarf aquarium lily, will root themselves firmly into the undergravel filter plates, so just keep that in mind when choosing placement for these plants. You may want to keep these deep-rooted plants in their original placements long-term, or placing them in spots with no undergravel plates, since they will be more difficult to remove and/or gravel vacuum around than plants with small root systems.

We recommend leaving areas under large hardscape or very densely planted areas free of undergravel filter plates, to prevent waste from building up in areas where you cannot gravel vacuum. That being said, you can absolutely have a beautifully aquascaped aquarium using an undergravel filter, it just requires a bit more strategic planning before putting everything together.

Which Substrate Should I Choose for an Undergravel Filter?

Choosing a type of gravel for use with an undergravel filter is wonderfully simple. Gravel that is at least 5mm on average will sit on top of the grates without falling through, making this the ideal size. Whether you choose a decorative, color-coordinated gravel from your local aquarium shop, or an inexpensive bulk bag of pea gravel from the hardware store, you have plenty of options that will look great, and function well with this type of filter. Keep in mind that the more textured the gravel is, the more surface area it will have for beneficial bacteria to grow.

How Many Undergravel Filter Kits Will I Need?

Each kit contains 20 Easy Undergravel Filter Plates. This means that one kit can work with 10 gallon tanks, 20 gallon, 29 gallon, and more. 9 plates can cover the bottom of a 10 gallon aquarium, which means that one kit can be utilized for two 10 gallon aquariums with one uplift tube for each. For larger tanks where you are wanting full coverage, we recommend two kits.

Each plate measures 5.5” x 2.75”. You can use the following formula to find out how many plates you need to cover the entire bottom of a larger aquarium:

Length of tank in inches, divided by 5.5 (round the result down to the nearest whole number) = A
Width of tank in inches, divided by 2.75 (round the result down to the nearest whole number) = B
Multiply A x B and that will give you the number of plates needed to cover the entire bottom of the aquarium.

If you only plan to cover a portion of the bottom of the aquarium, only measure the area you want undergravel plates to cover.

How to Set Up Your New Undergravel Filter

Parts needed to build your undergravel filter:

In addition to your undergravel filter parts, you will also need the following:

— Air pump

— Airline tubing

— Check valve (to prevent water from siphoning out of the tank when the power is off)

First, start with a clean, dry, bare-bottom aquarium, and figure out what kind of layout you’re going to want. You can set the plates up in a variety of different ways, and you can even leave certain areas open if you plan to place large hardscape or plants that will be difficult to gravel vacuum around. Line up and press the dovetail joints together from the top-down, for each plate you want to assemble:

Note that the plates will fit together lengthwise, as shown in the above photo, or they will fit perpendicular to one another, as shown below:

Then, you can assemble the uplift tubing. If your tank is tall, you may need to use the optional “A” tubes, with their corresponding “E” and “F” fittings, to increase the overall height. For shorter tanks, you may need to trim one of the “A” tubes, and the “B” tube, to fit. Shortened uplift tubes can make this undergravel filter fit an aquarium with as few as 8” from bottom glass to water line. You can see detailed instructions for how to assemble and/or trim the uplift tubing in this video here. The video depicts assembly of our sponge filter, but the main uplift tubing parts and assembly for the sponge filter is the same, making these parts interchangeable. To skip to the section that shows how to shorten the tubing, find timestamp 5:38.

Use a 1” piece of standard airline tubing to install the air stone (with one disc removed) into part “D”, it should look like this:

Next, install your uplift tubing on to the adapter piece, with the airline tubing attachments facing the back, so you can hide them more easily:

Once you have your plates set up and your uplift tubing assembled, choose where you want your uplift tubes to go, and slide the uplift-to-plate adapter on to the dovetail tracks on each long side of the plate, as shown below:

You can push the adapter along the rails to align it with the location you want it to stay in after your hardscape is installed.

You can then place your assembled undergravel filter into your aquarium if you have not already, and start placing your gravel over the plates, and hardscape where you want it. We recommend 1-2” of gravel across the entire plate area. Water will tend to follow the path of least resistance, so there will be greater flow through thinner areas of gravel.

Finally, install your airline tubing, check valve, and air pump.

If you are using both uplift tubes, we recommend using the dual outlet air pump. The single outlet air pump can be used if you are only utilizing one uplift tube from the kit.

Once you have installed the remaining equipment, and filled the aquarium, you can start the pump and adjust flow volume, as well as the direction of the curved uplift tube to create the directional flow you want in your tank.