Even with a new filter, aquariums still need regular maintenance, such as rinsing your filter media with waste tank water to preserve your aquarium's internal ecosystem, including the beneficial bacteria contributing to the nitrogen cycle. But the right filter can help establish a baseline for your tank's living inhabitants, maintaining clear water and enhancing aeration.
We tested the most popular internal aquarium filters, which mount on the inside of your tank, unlike external filters. The right aquarium filter will depend on your individual set-up—for example, the most powerful filter may not be appropriate for a small betta tank—so we looked for filters with the best combination of build-quality, features, and quiet operation.
Our tester tried multiple filters over four weeks, mounting each on the inside of a large aquarium tank and evaluating for ease of setup, noisiness, durability, and how easy they were to clean.
Here are our picks for the best internal aquarium filters, based on our intensive testing.
Fluval U2 Underwater Filter
Best quality sponge
Easy to unmount and clean
Fluval is a quality brand for many aquarium filters. This underwater model is slightly tricky to align the aerator function optimally, but its adjustable flow and quality sponges made up for the finicky setup, which only took us around five minutes.
The Fluval U2 allows for a variety of setup options, making it ideal for a broad spectrum of tank configurations. The adjustable water flow is a particularly useful feature, allowing you to pick whether the filter sprays out its cleaned water from the top, bottom, or across a tall spray bar, minimizing the water pressure output—a perfect pairing for planted tanks. You can even mount the Fluval U2 horizontally, though this option is only really useful for very shallow tanks.
While the Fluval U2 Underwater Filter wasn't the quietest filter we tested, it was close. Plus, with its built-in aeration contributing to your tank's oxygenation it's possible to supplement with a quieter, less obtrusive bubbler.
It was also one of the easiest to clean, making maintenance a cinch. All it takes is tilting the filter forward in order to remove the U2 from its bracket. Then you simply lift the lid for instant access to the easily swappable filter media.The U2's build quality was also high, with our tester giving it the highest possible rating for durability.
Cleaning the sponge and adjusting the flow is easy, with clear instructions provided by the manufacturer. In our four-week testing window, the only issue encountered was needing to adjust the suction cups. But once fixed, there were no further issues. This was the most expensive model we tested, but our testing confirmed its value.
Best Overall Runner-Up
Penn-Plax Cascade Internal 400
Easy to install
Adaptor for spray bar and aerator
Easy to remove carbon
Strict water level requirement
No bottom stabilizer
The Penn-Plax Cascade Internal filter was a very close runner-up. The filter was very easy to set up and had the added bonus of an option for aeration and a spray bar. The aeration hookup allows you to oxygenate the outgoing flow of water from the aquarium, while the spray bar spreads filtered water over the surface like a fountain.
The Cascade's provided filter media was slightly lower quality than our top pick but had an optional carbon canister that could be added or removed easily. The flow adjustment on the top of the filter was a little fidgety, but overall, this is a good quality filter and worth the investment.
The biggest issue with this filter was the size. The Penn-Plax Cascade is available in models suitable for 10-, 20-, and 50-gallon tanks, though we found the mid-range model a little underpowered. Finding a permanent attachment seal also proved tricky, especially without anything stabilizing the filter from the bottom.
Best for Large Tanks
JackSuper Ultra Quiet Fish Tank 4-in-1 Internal Aquarium Filter
Adaptor for aeration and wave
No instructions for cleaning
Adaptor did not stay attached
The Ultra Quiet Fish Tank 4-in-1 Internal Aquarium Filter packs a punch. It has multiple chambers and the highest flow of the filters we tested, making it ideal for large tanks or messy goldfish tanks. The JackSuper filter comes in four sizes, accommodating tanks up to 500 gallons. It also came with a wave cycle pump, which we found to be more of a curiosity than a useful function, though anyone looking for a filter for an outdoor pond might find waves useful.
The high-flow, almost jet-like output from this filter makes it an inappropriate pick for tanks with smaller fish or species that commonly have swim impairments, such as bettas. Look for a lower flow model instead. Also, while it may be quiet relative to its high-flow output, the JackSuper Ultra Quiet 4-in-1 was the loudest filter we tested.
Installing the JackSuper 4-in-1 was easy, and, unlike other filters we tested, it never budged once in position. The only issue we encountered came from its optional outflow attachments, which can be swapped depending on whether you want additional aeration or to generate waves on the water's surface. But unfortunately, the high flow kept pushing out the different nozzles. While this won't affect the overall functioning of the filter, it was a nuisance. Once you figure out which nozzle works best for your setup, it might help to glue it in place.
Best for Betta Tanks
Tetra Whisper Internal Power Filter
Quietest filter tested
Hangs below water line—cannot fill tank entirely
No flow adjustment
The Tetra Whisper Internal Filter makes some compromises, but its particular weaknesses also make it a solid pick for bettas and other small tank (less than 4 gallons), low stock aquarium situations.
The worst part of the Tetra Whisper is its filter media, which replaces the dense sponge found in most other filters tested with mesh pads sprinkled with carbon. Whereas sponge filters can last a few years between changes, the Tetra Whisper's filter medium seemed far less durable. Our best recommendation is to replace them with a sponge filter cut to the correct size.
True to its name, the Tetra Whisper was the quietest model we tested since water burbles out of the top instead of spraying through a nozzle. While this makes for a pleasant, babbling brook quality, it also means the Tetra Whisper needs to be placed fairly precisely relative to the water level (it can also be configured to sit on the bottom of a tank). It won't work right if your aquarium is filled to the brim. This finicky positioning combined with poor installation instructions made the Tetra Whisper particularly aggravating to set up.
However, its quiet operation and low flow make this a non-aggressive filter suitable for bettas, small fish, and minimal aquarium setups. People with large fish species, high population tanks, or have animals that create lots of waste, like turtles, should look elsewhere.
Overall, the Fluval U2 Series Underwater Filter is an investment, but it's well worth it. This quality filter is well adapted to many different tank setups and will likely last the lifetime of your fish. Be sure to keep an eye on the water line and filter setting if you wish to use it with the aeration attachment, but this is not necessary for most smaller setups. We know it says to replace the filter media sponge every month, but a rinse in waste tank water should keep it in good operating order for longer, even after a water change.
For large tanks or goldfish tanks, we highly recommend the Ultra Quiet 4-in-1 Internal Aquarium Filter. Although "ultra quiet" proved inaccurate, this filter was the highest performer in flow and overall filtration capacity. This is the ideal underwater filter for messy goldfish. You can even set up the wave outflow to give them a little extra exercise.
4.7 to 5 stars: These are the best internal aquarium filters we tested, which we can recommend without reservation.
4.3 to 4.6 stars: We strongly recommend these filters, but they may lack in one or more qualities belonging to our top picks, such as a slightly less durable build quality or fewer features.
3.8 to 4.2 stars: While we still recommend these filters, pay close attention to the caveats, as they may be most appropriate for very specific configurations, or have a substantial flaw.
3.5 to 3.7 stars: These filters provide average performance and shouldn't be preferred over other picks, even if they make work as a temporary solution.
3.4 and below: We don't recommend filters with scores below 3.5, as they either provide inadequate performance or have flaws so substantial that the product becomes nearly unusable.
Are internal aquarium filters better than external filters?
Internal aquarium filters are not better or worse than external filters when looking at your overall tank health. The major issue with internal vs. external is you will get wetter when you do maintenance on your internal filters. As long as you properly maintain your filter, it does not matter whether it is internal or external—it is only a matter of preference.
Where should you place an internal filter?
The placement of an internal filter will depend on the manufacturer. As we noted in our testing of the above filters, some types are very particular about where they must be placed relative to the water line, while others can be placed anywhere below the surface. Read your included setup instructions carefully before installing any filter unit.
Should an internal filter make bubbles?
Although many internal filter units contain aeration adaptors, your filter does not have to make bubbles. Unlike most external filters, internal filters will often not have the cascading effect that brings additional aeration and oxygen to your aquarium. However, most systems will have oxygen incorporated at the surface of the air-water interface. If your aquarium is not packed with many large fish (or species like goldfish, which require 20 gallons per fish), you will not likely need additional aeration.
If you are unable to test your tank's oxygen levels, clinical signs of low dissolved oxygen are fish laying on the bottom and exhibiting increased respiratory effort. You can always add a secondary aerator if you are concerned about the oxygen levels in your tank. It is almost impossible to add too much oxygen since the excess will bubble off at the surface.
Why Trust The Spruce Pets?
After over nine years in private clinical pet fish veterinary practice, Dr. Jessie Sanders has seen countless setups and is intimately familiar with most major brands of filters. She has had the same Fluval filter in her tropical tank for over seven years, operating with the exact same sponge she started with. After four weeks of testing these various filters, complete with full setup and cleaning, she has a very good idea of what you can expect over the medium and long term. As with all filters, be sure to read the instructions carefully and complete regular maintenance for best function and performance.