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If you’ve been looking for a way to improve the biological filtration in your tank without taking up much space, an under-gravel filter may be just what you’re looking for.

Undergravel filters are exactly what they sound like, a filter seated below the gravel or other large substrate. They work by pulling water through the substrate, using the surface area of your substrate to colonize beneficial bacteria. Some undergravel filters use carbon filter cartridges while others do not, so they are always biological filters but they are not always chemical filters.

Undergravel filters work best in conjunction with another type of filtration that can catch large particles of waste and detritus. This will help reduce the build-up of waste products underneath or inside of the undergravel filter. They work best in tanks less than 55 gallons, but there are a few that make great additions to larger tanks.

We’ve taken care of these product reviews to make it easier for you to pick an undergravel filter that will work best for the size, shape, and waste needs of your tank!

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A Quick Comparison of Our Favorites

Image Product Details
Best Overall
Winner
Penn Plax Premium Under Gravel Filter System Penn Plax Premium Under Gravel Filter System
  • Adjustable height lift tubes
  • Replaceable carbon filter cartridges included
  • Air stones included
  • Best Value
    Second place
    Imagitarium Undergravel Filter Imagitarium Undergravel Filter
  • Freshwater or saltwater
  • Lift tubes and air stones included
  • Replaceable carbon filter cartridges included
  • Premium Choice
    Third place
    Lee’s 40/55 Premium Undergravel Filter Lee’s 40/55 Premium Undergravel Filter
  • One plate of crack-resistant plastic
  • Freshwater and saltwater safe
  • Lift tubes and air stones included
  • Aquarium Equip Undergravel Filteration Aquarium Equip Undergravel Filteration
  • Freshwater, saltwater, and reef safe
  • Unique design
  • Can be used with sand
  • Aquarium Equip ISTA Undergravel Filter Aquarium Equip ISTA Undergravel Filter
  • Cost-effective
  • Lift tube is adjustable
  • Can be used to improve current filtration system
  • The 6 Best Undergravel Aquarium Filters – Reviews 2021

    1. Penn Plax Premium Under Gravel Filter System – Best Overall

    Penn Plax Premium Under Gravel

    The Penn Plax Premium Under Gravel Filter System is the best overall undergravel filter we reviewed. This filter is available in five sizes for tanks from 5 to 55 gallons. Checking the measurements of the product versus the measurements of your tank before purchasing is wise to make sure you get a filter that is the right size to filter your tank and physically fit into your tank.

    This filter system includes filter plates that snap together easily, lift tubes with adjustable height, high pore air stones, and your first set of carbon filter cartridges that are made to last 6-8 weeks. This undergravel filter system is easy to set up and the filter cartridges are easy to change, making maintenance easier. The slats in the filter plates in this set are made to work with gravel, so they are large enough to allow adequate flow without the substrate dropping into the filter.

    This kit does not include an air pump or airline tubing, so you will need to purchase these items separately. Make sure to purchase an air pump that can handle the tank’s volume. A mini air pump isn’t going to cut it for a 50-gallon tank.

    Pros
    • Available in five sizes from 5-55 gallons
    • Filter plates snap together easily
    • Adjustable height lift tubes
    • Replaceable carbon filter cartridges included
    • Air stones included
    • Easy setup
    • Designed to work with gravel
    Cons
    • No air pump or airline tubing included
    • Plastic clips for the plates may be easily broken

    2. Imagitarium Undergravel Filter – Best ValueImagitarium Undergravel Filter

    For the best undergravel aquarium filter for the money, we like the Imagitarium Undergravel Filter. This filtration system is available in 10-gallon and 29-gallon sizes and can be used in freshwater or saltwater tanks, although it is not reef safe.

    This filter kit includes two plates that snap together, but the plates don’t need to be snapped together for the filter to work effectively because the plates can function side-by-side without being connected. This kit also includes lift tubes, air stones, and replaceable carbon filter cartridges.

    This undergravel filter is extremely cost-effective and accepts filter cartridges from other brands, making it much easier to find replacement cartridges when needed. This filtration system does not include airline tubing or an air pump.

    Pros
    • Best value for the money
    • Freshwater or saltwater
    • Plates can function side-by-side
    • Lift tubes and air stones included
    • Replaceable carbon filter cartridges included
    • Accepts filter cartridges from other brands
    Cons
    • No air pump or airline tubing included
    • Plates may be difficult to snap together
    • Only two sizes available

    3. Lee’s 40/55 Premium Undergravel Filter – Premium ChoiceLee's 40 55 Premium Undergravel

    The best premium undergravel filter is Lee’s 40/55 Premium Undergravel Filter, or any of the Lee’s undergravel filters. These filters are available in six sizes from 10 gallons all the way up to 125 gallons. This filtration system is freshwater and saltwater safe.

    Lee’s Premium Undergravel Filters are unique because they consist of one high-quality plastic plate that is made to resist cracking under the weight of an aquarium substrate. The installation is easy and only having one plate takes one step out of the process. This kit also includes two lift tubes that are not adjustable, air stones, and replaceable carbon filter cartridges. These cartridges are not able to be fully removed from the filtration system for people who prefer to not use them.

    This system will accept parts from other brands and as a bonus, it’s black, so it will blend into more substrates better than other undergravel filters that are white or blue. Air pump and airline tubing are not included.

    Pros
    • One plate of crack-resistant plastic
    • Available in six sizes up to 125 gallons
    • Freshwater and saltwater safe
    • Lift tubes and air stones included
    • Replaceable carbon filter cartridges included
    • Accepts parts from other brands
    • Black color blends in well
    Cons
    • No air pump or airline tubing included
    • Premium price
    • Lift tubes aren’t adjustable
    • Cannot function correctly without the carbon filters attached

    4. Aquarium Equip Undergravel FilterationUndergravel Filteration Bottom

    The Aquarium Equip Undergravel Filteration kit is a little different from the normal undergravel filter. This consists of a set of tubes and elbows that sit under the substrate and allow water to flow through them. This filtration system is available in 10-gallon and 55-gallon sizes. It is freshwater, saltwater, and reef safe.

    This type of undergravel filtration system can be used with air pumps or connected directly to other filtration systems, like canister filters. It is not made to function as the sole filter of an aquarium. This kit only includes the plastic tubes and elbows and does not include an air stone, air pump, or other types of equipment.

    A nice benefit of the Aquarium Equip Undergravel Filteration system is that it can be used with sand substrate, unlike most undergravel filters. Ideally, it should be used with a larger substrate, like gravel, to improve water flow.

    Pros
    • Freshwater, saltwater, and reef safe
    • Unique design
    • Can be used to improve current filtration system
    • Can be used with sand
    • Includes all needed plastic tubes, elbows, and joint to create lift tube
    • Black color blends in well
    Cons
    • Cannot be sole filtration
    • No air pump, air stone, or airline tubing included
    • Only available in two sizes
    • No place to put any types of filter cartridges or media

    5. Aquarium Equip ISTA Undergravel FilterAquarium Equip ISTA Undergravel

    The Aquarium Equip ISTA Undergravel Filter is cost-effective but only available in one size for small tanks up to 10 gallons. This undergravel filter can be used in freshwater and saltwater tanks.

    This product is somewhat different from most undergravel filters because it does not sit flat on the bottom of the tank. Instead, the plate is attached to short legs that raise the plate off the bottom, but there are no sides to prevent waste and substrate from getting underneath the plate, which can lead to waste buildup. The plate can be cut to fit oddly shaped tanks and will still function as long as the lift tube can still be attached. The kit also includes an adjustable height lift tube and a plastic air “stone”. This filter is best used with gravel.

    This undergravel filter can be connected to an air pump as well as other filtration systems, like canister filters and HOB filters. This filter will not function adequately as the sole filter in the tank and does not have carbon filters.

    Pros
    • Freshwater and saltwater safe
    • Plate can be cut to fid odd shapes and sizes
    • Cost-effective
    • Lift tube is adjustable
    • Can be used to improve current filtration system
    Cons
    • Small size
    • May collect waste underneath the raised plate
    • Air stone is made from plastic
    • Cannot be sole filtration
    • Does not have carbon filter cartridges

    6. uxcell Plastic Fish Tank Undergravel Filteruxcell Plastic Fish Tank Undergravel

    The uxcell Plastic Fish Tank Undergravel Filter is a cost-effective product for various sizes of tanks. This kit comes with 24 small plates that can be connected to form a larger plate. They can be put together side-by-side or end-to-end.

    This kit includes the 24 black plastic plates, a lift tube, and an air hose line attached to an air stone. It does not include an air pump or full-length airline tubing. While the plates are black, the joints of the lift tube are white, making this noticeable in an aquarium.

    This system can be connected to an existing filtration system like HOB or canister, but it should not be used as the sole form of filtration. It does not have any type of carbon or other chemical filtration.

    Pros
    • Size to fit with plates that snap together
    • Can be used to improve current filtration system
    • Black color blends in well
    • Cost-effective
    Cons
    • Cannot be sole filtration
    • Does not have carbon filter cartridges
    • Joints of lift tube are white
    • Only one lift tube and air stone included
    • No air pump or full airline tubing included
    • Plates cannot be trimmed to fit odd shapes

    Buyer’s Guide

    What to Consider When Choosing an Undergravel Filter for Your Tank:
    • Your Fish: The type of fish you have will help you pick an undergravel filter if you pick an undergravel filter at all! Fish that love to burrow or dig, like loaches and cichlids, are not ideal candidates for undergravel filters. To work safely and effectively, undergravel filters need to be buried at all times. In a tank with fish that do a lot of landscaping, your filter may frequently become uncovered, decreasing its efficacy.
    • Your Tank’s Bioload: Not only do the types of fish you have matter, but also how many fish and the size of those fish! A 40-gallon tank with 10 neon tetras is going to have a far lower bioload than a 40-gallon tank with 5 goldfish. Undergravel filters aren’t a great option for heavy bioload tanks, but they are a great addition to understocked, appropriately stocked, or low bioload tanks with small fish or invertebrates.
    • Your Tank’s Size: Undergravel filters are most effective in tanks that are smaller than 55 gallons, but they can be used in larger tanks. Just make sure you’re picking an undergravel filter that is the right size for your tank or, for a larger tank, get more than one! If you have the outlet space, it’s not going to hurt anything to have more than one undergravel filter in your tank. More filtration is almost always better than less. You will just make sure you’re cleaning and performing maintenance on both filters regularly to make sure you’re not encouraging the growth of bad bacteria.
    • Your Substrate: Most undergravel filters will only work effectively with gravel or pebbles as the substrate. Sand is often too fine and will fill up the space under the plate, decreasing the filter’s ability to pull water down through the substrate to colonize bacteria. If your substrate is too large, like river rocks, then an undergravel filter won’t be particularly effective either. A substrate that is large enough to not fall under the filter plate but small enough to have a high surface area is going to give you the most bang for your buck when it comes to undergravel filters.
    • Your Plants: If you have a tank that is heavily planted with plants in the substrate, then an undergravel filter will likely result in your plants becoming stunted or dying. An undergravel filter will get in the way of root growth and installing it may disturb your plants. Also, if you have plants that do not like to be moved once planted, like Crypts, then installing an undergravel filter after you’ve established your planted tank will stress and potentially kill your plants. If your tank is full of floating plants or Java ferns attached to driftwood, then an undergravel filter isn’t going to be any problem for your plants.
    • Your Filtration: This one is a little bit tricky since undergravel filters are a type of filter. However, these filters are just not particularly effective on their own. All undergravel filters are going to work best in conjunction with another type of filtration, but what other filtration you use is up to you. Most undergravel filters can be connected to another filtration system, improving the efficacy of the other system. They can be connected to just about any type of filtration system but will work best in conjunction with a HOB, powerhead, or canister filter. If your tank filtration is an undergravel filter and a sponge filter, you likely aren’t going to have the water circulation and waste collection your tank needs.
    What Other Items You Will Need to Set Up an Undergravel Filter:
    • Substrate: Undergravel filters require 2.5-3 inches of a substrate to work effectively. Generally speaking, 1 pound of substrate per gallon will give you 1-2 inches of depth, depending on the floor space in your tank. This means that when you purchase your substrate, you will potentially need to get 1.5-2 pounds per gallon. Also, it’s going to be a lot easier on you if you’re installing an undergravel filter at the same time you’re putting in the substrate. Otherwise, you’re going to be trying to dig through the substrate to get the undergravel filter in place.
    • Air pump: An air pump is going to be what powers the air stone in the undergravel filter system. It’s important to make sure you get an air pump that is the correct size for the size of the tank you have, otherwise, you will probably end up with a weak bubbler instead of a functioning filtration system.
    • Airline tubing: The heart and soul of every air pump is the airline tubing that connects everything, but most pumps don’t include airline tubing. Read the description or package for the air pump carefully to see if it comes with tubing and if so, if it’s long enough for your needs. Airline tubing is generally inexpensive and easy to find.
    • Other filtration: If your tank is established, make sure you have another type of filtration up and running. If your tank is new and cycling with no fish, then you can put in the undergravel filter and a second filter of another type a little later on. Just remember that an undergravel filter isn’t going to produce the same water currents that a HOB or canister filter will, so an undergravel filter on its own, especially in a larger tank, may lead to poor water conditions.
    When to Use an Undergravel Filter When to Use a Different Filter Type
    When cycling and establishing a new aquarium In tanks with fish that dig regularly
    After a cycle crash due to cleaning or chemicals In a planted tank with well-established or sensitive plants already in place
    When you want to improve the functionality of your current filtration system In a bare-bottom tank
    In a low bioload or understocked tank In an overstocked tank
    In a tank with gravel or pebble substrate In a tank with a sand substrate (tube-based undergravel systems are the exception)
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    Conclusion

    These reviews cover more than one type of undergravel filter, giving you options for what might work best in your tank. The Penn Plax Premium Under Gravel Filter System was our pick for the best overall product for its high-quality and high functionality, but the Imagitarium Undergravel Filter has a similar appearance and function for the best value. For a premium product, check out the Lee’s 40/55 Premium Undergravel Filter. This product has a clean, simple design made from high-quality materials.

    All of these products have the potential to work well for you, but only you know what your ideal vision for your tank is. Undergravel filters aren’t without their issues, but they make an excellent addition to an established filtration system. They can also be used when cycling a new tank to speed up the process of beneficial bacteria growth. Undergravel filters are easy to set up and easy to use, making them beginner-friendly, so don’t let their unusual appearance intimidate you.


    Featured image credit: Krysja, Shutterstock