How Does an Undergravel Filter Work? – We’ll Explain in Detail

This article highlights everything you need to know about undergravel filters, including how to install and clean one.
James Palmer
James Palmer
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Last updated: September 03, 2023
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The undergravel filter debate has been going on for years. If you’re, however, a fish lover, then you know it’s a critical element of your aquarium. A good filtration unit not only keeps your aquarium clean but also keeps the fish healthy.

There are various types of filtration aquarium filters, and one of the most popular types is the undergravel filters. They help you keep your fish tank free from toxins, are easy to set up, affordable, and highly efficient. In this guide, we’ll answer the most asked questions, including how does an undergravel filter works, how to install and clean one, as well as the pros and cons of having one. Let’s dive into it!

What Is an Undergravel Filter and How It Works

According to an article by the BBC Trusted Source Aquariums 'deliver significant health benefits' People who spend time in aquariums could improve their physical and mental wellbeing, a study has suggested. , watching fish swim in aquarium tanks has both mental and physical benefits. Data from a controlled study revealed that people who watched with more fish exhibited a lower blood pressure and had a lower heart rate. They were also more relaxed and seemed generally happy. So, yes. There are benefits of having a fish tank in the house.

An undergravel filter is a type of filter whose primary filtration method involves a substrate, in this case, gravel. It has several components that help with its efficiency: a pump for air, a pump for drawing water in, and powerheads.

An undergravel filter also comes with a grate that prevents the gravel from getting to the bottom of the aquarium, and in some cases, it comes with filter media. Most of the time, however, the gravel acts as the primary filter media, but you have to clean it once in a while.

You’re probably wondering, how does undergravel filter systems work in an aquarium? Well, let’s first look at the basics. When you feed your fish, they defecate and this fecal matter, along with several unwanted particles, sink to the bottom of the tank. If you don’t retrieve or treat them, they will eventually decompose and release nitrites and ammonia, which are toxic to the fish. This is where an undergravel filter comes into play. The gravel, which is often small rocks or pebbles (don’t use sand as it will pass through the grate), sits on top of a slatted base and lets the unwanted particles pass through and sit at the bottom. This filter, however, can’t work alone and requires a powerhead pump and air pump that push water through the gravel. These two elements are often sold separately.

Powerhead pumps vacuum water with the unwanted particles towards the bottom and then uses filter tubes to release clean water to the tank. Air pumps, on the other hand, use a line to pump air from the bottom of the tank. Once these air bubbles rise, they push the unclean water down towards the gravel for filtering. Whichever pump you go for, one thing is constant; it will facilitate the filtration of unclean water by creating some form of water movement.

Undergravel filters conduct filtration in several forms:

How Does an Undergravel Filter Work? - We'll Explain in Detail

Mechanical Filtration

This type of filtration is more hands-on. The gravel at the bottom may be aesthetically pleasing, but it’s a lot more than a cool look. It acts as the mechanical filter and captures all the large particles as the water circulates in the tank. You, however, need to manually remove them as they accumulate, either using an aquarium gravel cleaner or during a water change.

Biological Filtration

This filtration method is very eco-friendly and occurs in several stages. First, the gravel, located on the filter tube walls and the bottom of the tank, acts as the primary filter. It provides a surface for bacteria growth, and this bacteria breaks down the ammonia and nitrates from fish feces, keeping the tank clean.

Chemical Filtration

This type of filtration method uses activated substances to absorb any harmful chemicals released by the decomposition of unwanted particles, fish feces, and fish food. You should, however, note that only select fish tanks models come with a chemical filtration system. You can, however, use canister filters to take out the harmful substances.

Based on multiple reviews, the Penn-Plax Filt-a-Carb cativated carbon media cartridges are some of the best in the market. They’re a universal fit and can be used on most undergravel filters and can last about 2 to 4 weeks. They’re relatively affordable, and since they use activated carbon, you can also use them to eliminate unwanted color and absorb bad odors from your tank.

Pros and Cons of Undergravel Filters

Based on an article by the Telegraph Trusted Source A beginner's guide to filling your first fish tank Follow Pets at Home’s easy fish points guide to match species to your first fish tank and ensure a happy and healthy aquarium. , filters are critical to keeping your aquarium clean and should be one of the accessories to buy for your first fish tank. Before you buy an undergravel filter, you need to first analyze its benefits and drawbacks to determine whether it’s the right fit for your fish tank. Here’s a quick overview of the top pros and cons


  1. They’re versatile: undergravel filters can be used on all types of fish tanks, including a small fishbowl and a several gallon tank. You can also increase their functionality by getting accessories. For instance, you can use chemical filters to absorb odor and remove discoloration.
  2. They’re easy to operate: undergravel filters are low maintenance making them a popular option for home aquariums. They’re also easy to install, have no moving parts, and typically don’t require many accessories. The gravel is all the substrate it needs, and you can easily clean it up during your routine water changes.
  3. They’re basically invisible: these types of filters are stealthy and are barely visible under the gravel. They filter the unwanted substances from your fish tank without impacting its aesthetic appeal.
  4. How Does an Undergravel Filter Work? - We'll Explain in DetailThe relatively affordable: compared to other types of flyers, undergravel filters are inexpensive, and you can get some models for under $50. You can use them for your fish tank or other marine animals such as turtles.
  5. They’re highly effective: metabolite poisoning is one of the leading causes of death among aquatic animals. It occurs when the ammonia and nitrate levels in the aquarium accumulate, causing harm to the fish. Underground filters are known for their capability to convert this toxic waste to desirable microbial life.

Based on multiple reviews, the Penn-Plax Clear-Free premium undergravel aquarium filter is one of the most popular models in the market. It’s both freshwater and saltwater safe and can be used on 40 and 55-gallon aquariums. This undergravel filter is also easy to set up & maintain and comes with 2 carbon filter cartridges that aid in removing water discoloration and odors. It doesn’t impact the aesthetics of your fish tank as it hides from view.


Even though undergravel filters are highly reliable, they have several drawbacks.

  • There’s a slight chance that your fish, especially the small breeds, could get trapped underneath the filter.
  • Most aquatic plants don’t thrive in areas with too much water circulation, which undergravel filters are known for.
  • If you neglect to maintain or change your gravel, it could start producing toxic gasses, causing a dead zone in your aquatic tank.
  • They could be difficult to maintain in the long run because you have to clean the gravel regularly.

How Do You Install an Undergravel Filter?

Installing an undergravel filter is fairly simple, but you have to follow this step-by-step guide for maximum efficiency.

  • Make sure the filter is the feisty accessory you install in the tank, as it should be located at the bottom.
  • Place plates at the bottom of the tank laying flat, and then connect the filter tube. They will suck in the unwanted particles by acting as a vacuum.
  • Pour your gravel on top of the plates and tubes so that it can act as the mechanical filter and substrate. The gravel layer shouldn’t be too thick or too thin. Anything between 1.5 to 3 inches thick is fine.
  • Pour in some water to confirm that it’s flowing freely under the gravel. This will also help you confirm that you’ve put in the right amount of gravel. If it’s too much, there’ll be restricted water movement under the gravel, while if it’s too little, the file won’t stay in place.
  • Pour in more water up to the suggested level, and then let it settle for a couple of seconds. You should then turn on the pump and pay attention to the water movement. It should start bubbling from the filter tubes.
  • Optimize the pump rate to suit the natural environment of the aquatic animals in your aquarium.

Final Thoughts

Undergravel filters are a great purchase. They’re easy to install, low maintenance, and reduce the accumulation of toxic particles for the fish’s fecal matter or food. They are also highly effective and can be used with various accessories to enhance their efficiency. How does an undergravel filter work? Well, they create water movement to eliminate unwanted substances in the fish tank. You should, however, stick to a regular cleaning schedule to prevent the accumulation of toxins. This should, however, be a minor inconvenience because water changes are a regular aspect of owning an aquarium.


People who spend time in aquariums could improve their physical and mental wellbeing, a study has suggested.
A beginner
Follow Pets at Home’s easy fish points guide to match species to your first fish tank and ensure a happy and healthy aquarium.
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