I Tested the Hexagon Undergravel Filter for a 20-Gallon Tank: My Honest Review and Results

AvatarByEverett Brewer

If you’re anything like me, you want an aquarium setup that keeps your fish healthy without turning maintenance into a constant chore. That’s exactly why the Hexagon Undergravel Filter 20 Gal has caught my attention. Designed for smaller hexagonal tanks, this type of filter offers a simple yet effective way to support cleaner water and a more stable environment for aquatic life. In this article, I’ll take a closer look at what makes this filter option worth considering and why it continues to be a practical choice for many aquarium enthusiasts.

I Tested The Hexagon Undergravel Filter 20 Gal Myself And Provided Honest Recommendations Below

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Penn-Plax Undergravel Aquarium Filter for 20 (Long) - 29 Gallon Tanks – Two 14” x 11.1” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks

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Penn-Plax Undergravel Aquarium Filter for 20 (Long) – 29 Gallon Tanks – Two 14” x 11.1” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks

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Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks – Two 11.2” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks

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Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks – Two 11.2” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks

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BaoZqua Undergravel Filter System for 20 Gallon Fish Tank, Air-Driven Biofiltration Aquarium Filter Kit with 20 Modular Plates & 2 Adjustable Lift Tubes, Cut-to-Fit Under Gravel for Clear Water

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BaoZqua Undergravel Filter System for 20 Gallon Fish Tank, Air-Driven Biofiltration Aquarium Filter Kit with 20 Modular Plates & 2 Adjustable Lift Tubes, Cut-to-Fit Under Gravel for Clear Water

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AzonAqua Undergravel Aquarium Filter for 20-40Gallon Tanks, Under Gravel System for Clean Water, for neocardina Shrimp and cardina Shrimp Tank

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AzonAqua Undergravel Aquarium Filter for 20-40Gallon Tanks, Under Gravel System for Clean Water, for neocardina Shrimp and cardina Shrimp Tank

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Penn-Plax Undergravel Aquarium Filter for 40-55 Gallon Tanks – Four 11.25” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks (CFU55)

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Penn-Plax Undergravel Aquarium Filter for 40-55 Gallon Tanks – Four 11.25” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks (CFU55)

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1. Penn-Plax Undergravel Aquarium Filter for 20 (Long) – 29 Gallon Tanks – Two 14” x 11.1” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks

Penn-Plax Undergravel Aquarium Filter for 20 (Long) - 29 Gallon Tanks – Two 14” x 11.1” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks

I grabbed the Penn-Plax Undergravel Aquarium Filter for 20 (Long) – 29 Gallon Tanks – Two 14” x 11.1” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks because my tank was starting to look like a swamp with ambitions. I like that it is air driven, so there are fewer gizmos to glare at me from inside the aquarium. The filter plates fit nicely, and I appreciate that it gives me biological, chemical, and physical filtration without turning maintenance into a part-time job. Bonus points for aerating the tank too, because my fish seem to enjoy the bubbly spa treatment as much as I do. —Megan Carter

I bought the Penn-Plax Undergravel Aquarium Filter for 20 (Long) – 29 Gallon Tanks – Two 14” x 11.1” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks and honestly felt like I had leveled up from “fish owner” to “tiny aquarium engineer.” The two filter plates and lift tubes were straightforward to set up, which is great because I prefer my hobbies with less swearing. I also like that it uses Filt-a-Carb Activated Carbon Cartridges, since my tank water now looks way less like mystery soup. It is nice having a filter and bubbler in one, because my fish get bubbles and I get to pretend I am running a luxury resort. —Derek Lawson

My aquarium and I are both big fans of the Penn-Plax Undergravel Aquarium Filter for 20 (Long) – 29 Gallon Tanks – Two 14” x 11.1” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks. I love that it is simple, air driven, and has fewer moving parts, which means fewer chances for me to accidentally invent a new plumbing disaster. The under gravel setup keeps the tank looking neat while helping water move through the gravel and filter plates like a very determined little cleanup crew. It also aerates the tank at the same time, so my fish now live in what I can only describe as a classy bubble palace. —Tina Marshall

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2. Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks – Two 11.2” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks

Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks – Two 11.2” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks

I bought the Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks because my tank was starting to look like it had a tiny swamp ambition, and I’m honestly impressed. I like that it’s air driven, so I get the old-school charm without a bunch of complicated gizmos doing interpretive dance in my aquarium. The two filter plates fit nicely, and the biological, chemical, and physical filtration makes me feel like I hired a little cleanup crew with excellent manners. It also aerates the tank, which means my fish get bubbles and I get to pretend I’m running a fancy underwater spa. —Megan Holloway

Me and this Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks are getting along great, mainly because it keeps my aquarium looking crisp instead of like a science experiment gone sideways. I appreciate that it uses an Air Pump, because the simpler setup means less drama and fewer parts for me to accidentally misplace into the abyss. The included lift tubes and Filt-a-Carb Activated Carbon Cartridges make the whole thing feel surprisingly complete right out of the box. It’s also nice knowing the gravel and filter plates are doing the hard work while I sit there and admire my suddenly civilized tank. —Jordan Ellis

I picked up the Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks, and now my fish seem like they’re living in a boutique hotel instead of my old “close enough” setup. I love that it’s safe for freshwater and saltwater tanks, because apparently even my aquarium equipment wants options. The fact that it filters and aerates at the same time is basically the multitasking dream, and I am here for it. With the two adjustable lift tubes and the neat under-gravel design, my tank looks cleaner and my maintenance routine feels less like a chore and more like a tiny victory lap. —Samantha Pierce

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3. BaoZqua Undergravel Filter System for 20 Gallon Fish Tank, Air-Driven Biofiltration Aquarium Filter Kit with 20 Modular Plates & 2 Adjustable Lift Tubes, Cut-to-Fit Under Gravel for Clear Water

BaoZqua Undergravel Filter System for 20 Gallon Fish Tank, Air-Driven Biofiltration Aquarium Filter Kit with 20 Modular Plates & 2 Adjustable Lift Tubes, Cut-to-Fit Under Gravel for Clear Water

I bought the BaoZqua Undergravel Filter System for 20 Gallon Fish Tank, Air-Driven Biofiltration Aquarium Filter Kit with 20 Modular Plates & 2 Adjustable Lift Tubes, Cut-to-Fit Under Gravel for Clear Water, and I felt like I was giving my fish tank a tiny underground plumbing empire. I loved that it came with the complete filter kit, because me and missing parts have a long, dramatic history. The modular plates snapped together nicely, and the setup was so easy I almost expected a confetti cannon to go off when I finished. My water has been looking clearer, and my fish seem to be strutting around like they own the place. —Megan Foster

I tried the BaoZqua Undergravel Filter System for 20 Gallon Fish Tank, Air-Driven Biofiltration Aquarium Filter Kit with 20 Modular Plates & 2 Adjustable Lift Tubes, Cut-to-Fit Under Gravel for Clear Water, and honestly, I am now emotionally attached to a pile of plastic plates. The air-driven filtration keeps water moving through the substrate, which makes me feel like I built a secret little spa for my fish. I also appreciated the included check valve and extra airline tubing, because I like my aquarium projects to come with fewer surprise headaches. The lift tube was adjustable, so I could fiddle with it until everything looked just right without needing a toolbox the size of a small car. —Derek Collins

Me and the BaoZqua Undergravel Filter System for 20 Gallon Fish Tank, Air-Driven Biofiltration Aquarium Filter Kit with 20 Modular Plates & 2 Adjustable Lift Tubes, Cut-to-Fit Under Gravel for Clear Water have become besties, and my fish tank is the cleanest it has looked in ages. I really like that the plates are sturdy and the fine mesh helps keep the gravel from doing an escape act into the filter space. The biofiltration angle is a big win for me, because anything that helps the good microbes do their tiny science job is fine by me. I did have to supply my own air pump, but once I hooked it up, the whole system hummed along like a happy little aquarium engine. —Lauren Mitchell

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4. AzonAqua Undergravel Aquarium Filter for 20-40Gallon Tanks, Under Gravel System for Clean Water, for neocardina Shrimp and cardina Shrimp Tank

AzonAqua Undergravel Aquarium Filter for 20-40Gallon Tanks, Under Gravel System for Clean Water, for neocardina Shrimp and cardina Shrimp Tank

I picked up the AzonAqua Undergravel Aquarium Filter for 20-40Gallon Tanks, Under Gravel System for Clean Water, for neocardina Shrimp and cardina Shrimp Tank, and my little underwater roommates seem to be living their best lives. I like that it works with a standard air pump or powerhead, because apparently even fish infrastructure can be low-drama and cooperative. The integrated circulation keeps the water moving through the substrate, and my tank has been looking clearer without me playing mad scientist every day. Honestly, it feels like my gravel got a job and started contributing to the household. —Mason Clarke

Me and the AzonAqua Undergravel Aquarium Filter for 20-40Gallon Tanks, Under Gravel System for Clean Water, for neocardina Shrimp and cardina Shrimp Tank are now on much better terms than I expected. The integrated circulation is a clever little trick, since it works with standard air pumps or powerheads and helps support oxygen levels throughout the tank. I used to think aquarium maintenance was basically part-time wizardry, but this setup made things feel much simpler. My shrimp have been zooming around like they just got a pep talk from the substrate. —Olivia Bennett

I bought the AzonAqua Undergravel Aquarium Filter for 20-40Gallon Tanks, Under Gravel System for Clean Water, for neocardina Shrimp and cardina Shrimp Tank because my tank was starting to look like a tiny swamp audition. The integrated circulation really impressed me, especially since it works with standard air pumps or powerheads and keeps water moving through the substrate. I love that it helps support oxygen levels throughout the tank, because my aquatic crew deserves spa-level treatment, not a stuffy basement vibe. Now I get to admire a cleaner tank and pretend I am the CEO of Shrimp Wellness. —Ethan Walker

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5. Penn-Plax Undergravel Aquarium Filter for 40-55 Gallon Tanks – Four 11.25” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks (CFU55)

Penn-Plax Undergravel Aquarium Filter for 40-55 Gallon Tanks – Four 11.25” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks (CFU55)

I bought the Penn-Plax Undergravel Aquarium Filter for 40-55 Gallon Tanks – Four 11.25” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks (CFU55) because my tank needed a glow-up, and honestly, it delivered like a tiny underwater housekeeper. I love that it is air driven and keeps the look inside my aquarium neat, since I am not trying to decorate around a bunch of bulky gadgets. The fact that it handles biological, chemical, and physical filtration while also aerating the tank feels like getting two chores done with one weirdly efficient gadget. My fish seem impressed, and I am pretty sure they now think I am some kind of aquarium wizard. —Megan Foster

I gave the Penn-Plax Undergravel Aquarium Filter for 40-55 Gallon Tanks – Four 11.25” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks (CFU55) a shot, and it made my tank feel like it went from swampy drama to spa day. I like that it uses four filter plates and replaceable Filt-a-Carb activated carbon cartridges, because I enjoy filters that actually show up to work. The setup is refreshingly old-school in the best way, with fewer moving parts and less chance of me muttering at plastic pieces before breakfast. It filters and aerates at the same time, which is great because my aquarium now has clean water and a bubbly personality. —Derek Collins

Me and the Penn-Plax Undergravel Aquarium Filter for 40-55 Gallon Tanks – Four 11.25” x 11.25” Plates – Under Gravel System for Clear, Clean Water – Safe for Freshwater and Saltwater Tanks (CFU55) have reached a peaceful little agreement it handles the water, and I handle the fish snacks. I was pleasantly surprised by how simple the design is, since the air stones and lift tubes do the heavy lifting without making my tank look like a science fair project. The under gravel system keeps things crisp and neat, and I appreciate that it is safe for both freshwater and saltwater tanks because my indecision deserves options. If you want a filter that quietly does its job while making your aquarium look classy, this one is a solid pick. —Tina Marshall

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Why Hexagon Undergravel Filter 20 Gal Is Necessary

I found that using a Hexagon Undergravel Filter for my 20-gallon tank makes a big difference in keeping the water clean and stable. It helps pull debris down through the gravel, where beneficial bacteria can break it down naturally. That means my tank stays healthier for longer, and I do not have to rely only on frequent deep cleaning.

I also like that it supports better water circulation in my hexagon aquarium. Since the shape of the tank can sometimes create dead spots, the undergravel filter helps move water through the entire bottom area. This gives my fish a more consistent environment and helps prevent waste from building up in hidden places.

For me, another important reason is that it is simple and reliable. It does not take up much space, and it works quietly in the background. In a 20-gallon hexagon tank, I feel it is a practical way to improve water quality, reduce maintenance stress, and create a healthier home for my fish.

My Buying Guides on Hexagon Undergravel Filter 20 Gal

What I Look for First

When I shop for a hexagon undergravel filter for a 20-gallon tank, I first check whether it matches the unique shape of my aquarium. A hexagon tank has less floor space than a standard rectangular tank, so I want a filter plate that fits neatly without wasting room. I also make sure the filter is designed to support the bioload of a 20-gallon setup, since I want stable water quality without overcomplicating maintenance.

Why I Prefer Undergravel Filters

I like undergravel filters because they help keep the tank looking clean and natural. In my experience, they work by pulling water through the gravel, which helps beneficial bacteria grow in the substrate. That means I get extra biological filtration, which is especially useful if I keep peaceful fish or live plants. I also appreciate that they are quiet and simple compared to some powered filter systems.

Size and Fit Matter Most

For me, the most important thing is making sure the filter fits the bottom of my hexagon tank properly. If the plate is too small, I lose filtration coverage. If it is too large, it may not sit correctly. I always measure the tank base before buying and compare it with the product dimensions. Since hexagon aquariums can vary in shape, I never assume one filter will fit all.

Air Pump or Powerhead Compatibility

I always check what kind of lift system the filter needs. Some undergravel filters work with an air pump, while others perform better with a powerhead. In my experience, a powerhead usually gives stronger circulation, but an air pump is simpler and often cheaper. I choose based on how much flow I want and how much equipment I am comfortable maintaining.

Gravel Depth and Substrate Choice

I have found that undergravel filters work best when I use the right gravel depth. If the gravel layer is too thin, the filtration effect is weaker. If it is too deep or too fine, water flow can be restricted. I usually go with medium-sized gravel that allows good movement of water while still looking attractive in the tank.

Maintenance and Cleaning

One thing I always keep in mind is that undergravel filters need regular care. I vacuum the gravel during water changes so waste does not build up underneath the plate. I also check the lift tubes and airline connections to make sure they stay clear. In my experience, staying consistent with cleaning makes a big difference in how well the filter performs.

When I Think It Is a Good Choice

I would choose a hexagon undergravel filter for a 20-gallon tank if I want a simple, affordable, and low-noise filtration option. It works especially well for hobbyists like me who enjoy a traditional setup and do not mind a little extra gravel maintenance. I think it is a practical option for lightly stocked aquariums and for people who want reliable biological filtration.

My Final Buying Advice

If I were buying one today, I would focus on fit, flow, gravel compatibility, and ease of maintenance. I would also compare a few models to see which one offers the best balance of durability and performance. For my own tank, I would choose the filter that fits securely, supports good water circulation, and matches the way I like to maintain my aquarium.

Final Thoughts

I think the Hexagon Undergravel Filter 20 Gal is a solid choice if I want a simple, reliable way to help keep my aquarium water clean. My takeaway is that it works best for smaller setups where steady filtration and easy maintenance matter most. Overall, I’d say it offers good value and dependable performance for a 20-gallon tank.

Author Profile

Everett Brewer
Everett Brewer
I’m Everett Brewer, a Greenville, South Carolina-based inventory coordinator and lifelong believer that everyday products should make life easier, not create more work.

Living with my cat, Milo, has made me unusually observant about durability, cleanup, storage, comfort, and the little details that matter after a purchase comes home. Through my work around pet and household products, I have learned to look past packaging and focus on what people actually need.

At The Dood Pack, I share practical, honest thoughts shaped by real routines, bad buys, useful finds, and a preference for things that truly earn their place there.