How Does An Undergravel Filter Work?

If you’re setting up a new aquarium, there are a bunch of decisions you need to make long before the first fish hits the water. Among them is deciding what kind of filter is right for your tank.

There are many different kinds of filter to choose from, each with its pros and cons.

While we can’t tell you for sure what’s best for you, we can go over some of your options to help you make an informed decision.

Some neon tetras in a gravel bottomed, planted tank

For this article, we’re looking at the under gravel filter, or UGF for short. We’ll cover what they are and how undergravel filters work, before discussing the advantages and disadvantages relative to other filtration systems.

By the time you’re finished reading, hopefully, you’ll know whether or not to filter this one out of your list of possibilities.

What Is An Undergravel Aquarium Filter?

Since you’ll never potty train your fish, they will do what they do naturally – let it go right in the water. Natural bodies of water have natural filters to take care of all that stuff, like sandy bottoms, inflowing and outflowing rivers and streams, bottom-feeding fish, etc.

Every aquarium needs a filter to do what nature would do – keep the water safe and clean for its inhabitants. There are many types, but they all do essentially the same thing.

As the name suggests, an undergravel filter sits at the very bottom of your tank, underneath the substrate.

Most often, they resemble a tray that matches the rectangular shape of your tank. This prevents most of the gravel from touching the bottom. Others resemble a grid of pipes, and while they allow gravel to touch the bottom, they can still be just as effective.

Undergravel filters mainly provide biological filtering, but may also provide just a smidge of mechanical filtering. They also turn the gravel itself into a very effective mechanical filter.

How Does An Undergravel Filter Work?

Because it’s beneath the substrate, a UGF allows water to flow at the very bottom of the tank. Normally, this is a place of complete stagnation where fish poop and uneaten food can rest in peace.

Plate-style UGFs (we need an acronym to keep this brief) are perforated with holes. Tube-style UGFs have holes drilled around the tubes throughout the system. A lift tube ascends the tank from the bottom of the filter to a point at or near the surface of the water.

Some models utilize an air pump to blow bubbles down an airline to the bottom of the lift tube. This creates suction at the bottom of the lift tube, and pulls water through the plate or grid, up the tube and back into the tank. Other models employ a power head to draw water up the lift tube without bubbles.

By drawing the aquarium water down through the gravel, all the waste in the tank is pulled to the bottom of the tank. Large particulate is caught in the gravel, and small, even microscopic gunk makes its way to the filter.

Over time, good bacteria will colonize not only the gravel but also the UGF. Without one, the tank bottom under the gravel becomes an anaerobic zone.

This means there’s no oxygen present, so nothing that needs O2 to survive will exist there. By pulling water through the gravel, oxygen is introduced to the area, allowing bacteria to move in.

The bacteria, as part of the Nitrogen Cycle, will convert ammonia (a common byproduct of fish waste) into nitrite, and then into relatively harmless nitrate. This is a very important part of keeping a tank clean.


No matter how many people claim it, there’s no such thing as a “maintenance-free” aquarium. There’s always something to do!

As filters go, UGFs are pretty easy to take care of. The filter itself doesn’t collect much of any solid waste, so there’s no media to rinse or replace. It is possible that, given enough time, the holes could become clogged with fine matter. In that case, it would be necessary to remove the entire filter and give it a good rinsing.

Generally, though, the only thing you’ll need to do is give the gravel a thorough vacuuming now and then to remove all the large bits of debris. This will allow the water to keep flowing through the gravel and the filter system. If you do this regularly, you likely won’t ever need to remove the whole system for cleaning.

Pros and Cons of Undergravel Filters

Every type of filter has positive and negative attributes. Here are a few for UGFs.


  • Low maintenance
  • No media (like charcoal) to replace
  • Promotes biological filtration
  • Inexpensive
  • Totally hidden (except the lift tube, which is clear anyway)


  • Vacuuming needs to be thorough and regular
  • Shouldn’t be used with fish that like to dig
  • Typical UGFs may not be suitable for tanks with plants
  • If you do have to remove it, it’s a big job

What Kind of Tank Are They Best For?

Really any small to medium tank is suitable for a UGF, even if it’s just one of multiple filter systems you use. If you have a very large tank, a UGF is not going to be enough on its own, and you might have difficulty finding one to fit, anyway. If that’s the case, you may be a candidate for a DIY version. (See below to learn more.)

If you were planning to use sand as substrate, a UGF isn’t for you. You have to use coarse gravel with a UGF to prevent the substrate from being drawn through the filter.

And, as mentioned in the cons in the previous section, fish that like to dig (like loaches) are not suited for a tank with a UGF. Fish that burrow create channels through the gravel and will disrupt the even flow of water, thus hindering the filtration process.

Are They Good for Planted Tanks?

Many aquarists love to incorporate live plants in their tanks. As mentioned earlier, plants and traditional UGFs don’t always get along. Their roots can clog the intakes, and the flowing water may cause fluctuations in oxygen levels that they don’t like.

What about a non-traditional UGF? If you love plants, consider a reverse undergravel filter. It is not difficult to set up a system in which the water is pushed through the plate instead of drawn in. This brings fresh, clean, oxygenated water down to the very tips of your plants roots.

Check out this video that shows beautifully planted aquariums that use undergravel filters to help the plants thrive. WARNING: there’s some pretty relaxing music playing and you might find yourself lulled by the plants, fish, music, and the narrator’s voice into a state of semi-hypnosis.


DIY – Building, Instead of Buying One of Your Own

Although there are several commercially available undergravel filters on the market, some handy folks like to build their own. A homemade one can be made to fit to the exact dimensions of your tank, even if it’s not a standard rectangle. They aren’t that hard to make, and you get the satisfaction only DIYers know.

Most DIY UGFs are made from small diameter PVC piping that you can easily purchase at any home building and renovation center. A quick search of the ‘net will turn up lots of helpful videos. It really looks like a fun and simple project!

Final Thoughts

If you search around the internet, you’ll find a few sources telling you that the undergravel filter is yesterday’s technology and has no place in a modern aquarium. Others, however, swear by the simplicity and the fact it’s a tried-and-true solution for a universal problem.

Our thought is that it’s the perfect filter for specific applications. Only you can decide if it’s right for you.

Thanks for reading, and please take the time to share the link with anyone you know in the aquarium community who might find it useful. We’re happy to be of service!

Happy fish keeping!

B Hamilton

Hey there! I'm Brian, a lifelong enthusiast and fish keeper with a wealth of knowledge and experience on freshwater aquariums, that I love to share on this site. If you have any questions or need any help, please do ask in the comments section below, I'd love to hear from you and will help where I can.

3 thoughts on “How Does An Undergravel Filter Work?”

  1. Can you use an outside filter with an under gravel filter ?

    • Hi Ken. Not sure what you mean, can you clarify with more info? What set up exactly are we talking about?

  2. I think Ken wants to double up on filtration. A hang-on-back teamed with the substrate filter. In which case I don’t see any problem with superior water clarity.

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