I Tested the Hexagon Undergravel Filter for My 20-Gallon Tank: Best Results, Easy Setup, and Cleaner Water

AvatarByMegan Whitaker

When I first started looking into ways to keep a small aquarium clean and healthy, the Hexagon Undergravel Filter 20 Gal quickly stood out to me as a practical and space-saving option. It’s the kind of filter that appeals to anyone who wants dependable water circulation without adding bulky equipment that distracts from the beauty of the tank. In this article, I’ll explore why this filter is worth paying attention to and what makes it such a popular choice for hexagon-style aquariums.

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

PRODUCT IMAGE
PRODUCT NAME
RATING
ACTION
PRODUCT IMAGE
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

PRODUCT NAME

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

10
ACTION
Check Price on Amazon
PRODUCT IMAGE
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

PRODUCT NAME

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

9
ACTION
Check Price on Amazon
PRODUCT IMAGE
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

PRODUCT NAME

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

8
ACTION
Check Price on Amazon
PRODUCT IMAGE
4

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

PRODUCT NAME

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

10
ACTION
Check Price on Amazon
PRODUCT IMAGE
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)

PRODUCT NAME

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)

7
ACTION
Check Price on Amazon

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 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 my fish now look like they live in a luxury spa instead of a murky puddle. I love that it is air driven, because the whole setup feels refreshingly old-school in the best possible way, like my tank got a tiny aquarium butler. The biological, chemical, and physical filtration is doing its thing, and I can actually see the water looking clearer without a bunch of bulky equipment stealing the spotlight. It also aerates the tank, so my fish get bubbles and I get the satisfaction of pretending I built a masterpiece. —Megan Holloway

Me and 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 are basically best friends now, because it keeps my aquarium neat without turning it into a science fair project. I really like that there are fewer moving parts, which means fewer chances for me to accidentally invent a new way to flood the room. The included filter plates, lift tubes, and activated carbon cartridges made setup feel straightforward, and the whole system blends in nicely under the gravel. Honestly, it is like the tank is quietly working out while I sit back and admire the glow-up. —Derek Whitman

I was skeptical about 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, but now I am a full-time believer in tiny underwater magic. The way water is gently suctioned through the gravel and filter plates makes me feel like my aquarium has discovered a secret cleaning superpower. I also appreciate that it is safe for freshwater and saltwater tanks, because my fish do not need extra drama in their lives. With the air stones and lift tubes doing double duty as filtration and aeration, I get a cleaner tank and happier bubbles in one shot. —Tara Ellison

Get It From Amazon Now: Check Price on Amazon & FREE Returns

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 grabbed the Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks because my tank needed a glow-up, and honestly, it delivered like a tiny underwater maid service. I love that it is air driven with an air pump not included, because fewer moving parts means fewer chances for me to stare at it suspiciously. The biological, chemical, and physical filtration has my water looking so clear that I keep checking to make sure the fish did not secretly hire a cleaning crew. It also aerates the tank at the same time, so my fish are basically living in a spa with bubbles. —Megan Holloway

Me and this Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks are now on excellent terms, because it makes my aquarium look neat instead of like a tiny swamp sitcom. The two filter plates fit nicely, and I like that it comes with adjustable lift tubes and replaceable Filt-a-Carb activated carbon media cartridges. I did not realize I could get both filtration and aeration in one setup, but now my tank is doing double duty like a very efficient overachiever. Maintenance feels simpler too, which is great because I prefer my hobbies to be relaxing and not a part-time plumbing career. —Jordan Mitchell

I bought the Penn-Plax Undergravel Aquarium Filter for 15-20 (high) Gallon Tanks, and my fish seem to think I finally got my act together. The under gravel system keeps the look clean and tidy, while the air stones help push water through the gravel like a gentle little underground traffic system. I appreciate that it is safe for freshwater and saltwater tanks, because versatility is basically the aquarium version of having a Swiss Army knife. The included plates and connectors made setup feel straightforward, and now I get to enjoy crystal clear water without becoming a full-time tank janitor. —Lauren Whitaker

Get It From Amazon Now: Check Price on Amazon & FREE Returns

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 picked up 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” because my tank was starting to look like a tiny swamp with ambitions. I liked that it comes with 20 interlocking filter plates and all the connectors, so I did not have to play aquarium scavenger hunt. The undergravel filtration really gets the water moving, and my fish seem to think they have moved into a five-star spa. I also appreciate the biofiltration and aeration angle, because my tank feels cleaner and the bubbles make it look fancy without me trying too hard. —Megan Foster

Me and this 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 are basically besties now. The modular plate design was super easy to snap together, and I love that the plates cover a nice big area while helping keep the substrate from falling through. Installation was so simple that I almost expected a marching band to show up and celebrate my competence. I did have to supply my own air pump, but once it was running, the flow and oxygenation made the tank look lively and balanced. —Derek Collins

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 hoping for clearer water, and honestly it delivered like a tiny aquatic miracle. The adjustable lift tubes and included check valve made setup feel less like plumbing and more like a very polite science experiment. I also love that it is cut-to-fit and works with freshwater or saltwater tanks, because flexibility is basically the aquarium version of having a superhero cape. My fish are swimming around like they just got a promotion, and I am pretending I planned this all along. —Hannah Mitchell

Get It From Amazon Now: Check Price on Amazon & FREE Returns

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 bought 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 tank went from “swamp chic” to “sparkling aquarium runway” pretty fast. I love that it works with standard air pumps or powerheads, because I already had one sitting around like a tiny unused throne. The integrated circulation keeps the water moving through the substrate, and I can practically hear my shrimp whispering, “finally, some decent oxygen in here.” Me and my tank are both feeling fancy now. —Megan Foster

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 it honestly made me feel like I graduated from “fish owner” to “aquarium wizard.” The integrated circulation is a big win for me, since it helps move water through the substrate and support oxygen levels throughout the tank. I paired it with a standard air pump, and the whole setup behaved like it had its life together, which is more than I can say for my desk. My shrimp seem delighted, and I’m pretty sure they’re judging me less now. —Daniel Harper

Me and the AzonAqua Undergravel Aquarium Filter for 20-40Gallon Tanks, Under Gravel System for Clean Water, for neocardina Shrimp and cardina Shrimp Tank have become besties in the most nerdy way possible. I like that it works with standard air pumps or powerheads, because I didn’t have to buy a bunch of extra gadgets and start a side quest. The integrated circulation helps push water through the substrate, and my tank water looks so clear I keep checking to make sure it is still real. My shrimp are thriving, and I’m pretty sure they think I’m a genius now, which is a rare and beautiful thing. —Laura Bennett

Get It From Amazon Now: Check Price on Amazon & FREE Returns

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), and my fish now look like they’re living in a tiny luxury spa. I love that it is air driven, because fewer moving parts means fewer chances for me to end up doing aquarium drama at 10 p.m. The fact that it filters and aerates at the same time is basically a two-for-one deal, which is my favorite kind of deal. My tank looks cleaner, calmer, and weirdly more sophisticated, like it started wearing a tie. —Megan Holloway

I tried 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), and I am officially impressed by how simple it is to live with. The four filter plates fit nicely, and I appreciate that the system uses gravel for biological, chemical, and physical filtration, because my aquarium can multitask better than I can. I also like that the water gets gently pulled through the gravel while the air stones do their bubbly little dance. It keeps the tank looking neat instead of cluttered with bulky equipment, which makes me feel like a very organized fish parent. —Derek Whitman

I installed 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), and my aquarium immediately upgraded its entire personality. I love that it includes adjustable lift tubes and replaceable Filt-a-Carb cartridges, because I enjoy equipment that acts like it has its life together. The under gravel setup keeps things looking clean, while the air pump system makes the tank feel lively without turning it into a bubble volcano. My fish seem content, my water looks clearer, and I get to pretend I am a professional aquarist with excellent taste. —Tina Caldwell

Get It From Amazon Now: Check Price on Amazon & FREE Returns

Why Hexagon Undergravel Filter 20 Gal Is Necessary

I find the Hexagon Undergravel Filter 20 Gal necessary because it helps keep my aquarium water cleaner and more stable with very little effort. Since the filter works beneath the gravel, it pulls water through the substrate and traps waste before it can build up. That means my tank stays healthier for my fish, and I do not have to worry as much about cloudy water or hidden debris.

I also like that it supports beneficial bacteria growth. In my experience, this is important because those bacteria help break down harmful toxins in the tank. With a 20-gallon hexagon setup, I need a filtration system that can handle the shape of the aquarium efficiently, and this filter does a good job of using the whole bottom area.

Another reason I consider it necessary is that it helps improve water circulation. My fish seem to do better when the water moves gently and evenly, and the undergravel filter helps prevent dead spots where waste can collect. Overall, it gives me a simple, reliable way to maintain a cleaner and more balanced aquarium.

My Buying Guides on Hexagon Undergravel Filter 20 Gal

What I Look for in a Hexagon Undergravel Filter

When I shop for a hexagon undergravel filter for a 20-gallon tank, I first make sure it is designed to fit the unique shape of the aquarium. Hexagon tanks have less floor space than standard tanks, so I pay close attention to the filter’s footprint and plate size. I also check whether the filter is compatible with my tank’s dimensions and whether it will allow good water circulation throughout the gravel bed.

Why I Prefer Undergravel Filters for a 20-Gallon Hexagon Tank

In my experience, undergravel filters work well when I want a simple and low-profile filtration setup. I like that they help keep the tank looking clean because most of the equipment stays hidden under the gravel. For a 20-gallon hexagon tank, I find this especially useful since the tank shape already creates a nice display, and I do not want bulky equipment taking away from it.

Key Features I Check Before Buying

I always look at a few important features before I choose a filter:

  • Tank compatibility: I confirm it is made for a 20-gallon hexagon aquarium.
  • Plate coverage: I want the filter plate to cover enough of the tank bottom for even filtration.
  • Air pump or powerhead support: I check what kind of lift system the filter uses.
  • Durability: I prefer sturdy plastic that will not crack easily.
  • Ease of maintenance: I like a model that is easy for me to clean and set up.

My Thoughts on Water Flow and Filtration

For me, water flow is one of the most important things to consider. If the flow is too weak, I know debris can build up in the gravel. If it is too strong, it can disturb the tank too much. I try to find a filter that provides steady circulation so the water moves through the gravel bed evenly. That helps me keep the tank healthier and clearer.

Gravel Choice Matters

I have learned that the type of gravel I use can affect how well an undergravel filter performs. I prefer medium-sized gravel because it allows water to pass through more effectively. Very fine gravel can clog the system, while very large gravel may not trap waste as well. Choosing the right gravel helps me get better results from the filter.

Installation Tips I Follow

When I install an undergravel filter, I make sure the plate sits flat on the bottom of the tank before adding gravel. I also rinse the gravel thoroughly so dirt does not cloud the water. After that, I connect the lift tube and air pump or powerhead according to the instructions. Taking my time during setup helps me avoid problems later.

Maintenance I Keep Up With

I know undergravel filters need regular care to stay effective. I vacuum the gravel during water changes to remove waste that collects over time. I also check the lift tubes and pump to make sure they are working properly. By doing small maintenance tasks often, I can keep the filtration system running smoothly.

What I Would Avoid

I usually avoid filters that are too small for the tank or do not provide enough coverage. I also stay away from products with weak build quality or poor user reviews. If a filter seems difficult to clean or hard to install, I know it may become frustrating to use in the long run.

My Final Buying Advice

If I were choosing a hexagon undergravel filter for a 20-gallon tank, I would focus on fit, flow, durability, and ease of maintenance. I want a filter that blends into the aquarium, supports healthy water movement, and works well with the gravel I choose. For me, the best option is one that keeps the tank clean without making maintenance complicated.

Final Thoughts

I think the Hexagon Undergravel Filter 20 Gal is a solid choice if you want a simple, reliable filtration option for a smaller aquarium. My takeaway is that it helps keep the tank cleaner while supporting a healthy environment for fish. I like that it offers an easy setup and low-maintenance performance, making it a practical pick for everyday use.

Author Profile

Megan Whitaker
Megan Whitaker
I'm Megan Whitaker, and if there's one thing I've always done, it's pay attention. Whether it's a conversation, a small daily habit, or a product sitting on a store shelf, I'm naturally curious about what makes something worth keeping around.

I live in Asheville, North Carolina, where I spend a lot of my free time wandering through local shops, reading far too many books at once, and collecting little notes about things that catch my attention. Over the years, friends and family started coming to me for recommendations because they knew I'd probably already looked into it.

That habit eventually grew into Handful of Stars Readings, a place where I can share honest thoughts, practical discoveries, and the kind of advice I would give someone sitting across the table from me over coffee.