Aquariums are great décor, when you’ve got the space. Unfortunately, the accessories themselves can make space management more difficult, especially filters.
Whether they’re hanging on the back or running a ton of tubes into the tank, external filters need a lot of space. But, internal filters don’t.
The best internal fish tank filter will sit perfectly inside your tank. You can shove that tank right up to the edge of the wall without leaving 6 inches on all side for the needy filter.
Learn more about internal filters, how to choose a good one, and see our reviews of a few of our favorites…
- 8 Top-Rated Internal Fish Tank Filters To Buy In 2021
- What To Look For In An Internal Tank Filter
8 Top-Rated Internal Fish Tank Filters To Buy In 2021
1 Aqueon Quietflow Internal Power Filter
There are 4 different sizes available for this filter. It comes in 10, 15, 30, and 40-gallon models. Each model has slight size differences based on the flow rate.
Aqueon’s Quietflow internal power filter is a fully submersible filter that sits anywhere in the tank. Place it vertically or horizontally, as long as it’s fully underwater. It shouldn’t stay on while out of water.
This filter has all 3 stages of filtration. It uses cartridges that are replaceable and easy to find. Once you know the cartridge size, which may change between filter sizes, you can buy any cartridge with similar dimensions.
Change the direction of the water return valve if the flow rate is messing with your tank. It can be shortened, lengthened, and re-directed as needed. Flow rate can also be adjusted lower if it’s too strong for your fish.
If all you need is a simple internal filter in a small to medium tank, you’ll probably be happy with this Aqueon model. It doesn’t have any bells or whistles or complex features to worry about, making it a quick, easy solution for clean water.
Want the full picture? Let’s spill the tea…
2 Penn Plax Cascade 300
The Penn Plax Cascade 300 is a fully submersible internal filter with a transparent blue bottom section and a black top. It’s available in packs with 1 to 5 full filters per pack.
This filter’s flow rate is 70 GPH. It’s meant for tanks around 10 to 15 gallons in size. You can place it anywhere in your tank, horizontally or vertically, as long as it’s mostly submersed. The flow rate is adjustable.
Rather than using cartridges, this filter has a high capacity filtration media cup that accepts loose media. You can fill it with whatever you want and get the right level of filtration for your tank. It works well with sponges and most biological and chemical filtration media.
For the output, you can have the top water output spout hanging above the water for aeration. Or, you can place it below the water to evenly distribute the water. Water output doesn’t create any undesired currents in the tank.
This submersible filter gives you more options for filtration media. It’s a good choice if you want more flexibility instead of using only specific cartridges. The flow rate is also strong enough to do well in small tanks without struggling.
Look at the pros and cons of this filter…
3 Marina I25 Internal Filter
The Marina I25 filter is a small, black clip-on filter for tanks up to 6.6 gallons. It’s a good alternative for small tanks that don’t have a lot of space for filtration. The GPH is a maximum of 26.4.
While this is an internal filter, it must sit vertically. You cannot place it horizontally, because of the interior water flow system. Water comes in through the bottom and trickles out from the top.
A cartridge sits in the center of the filter. Each filter contains mechanical, biological, and chemical filtration. Water enters in the center of the cartridge, giving it more time to interact with most of the media inside before it trickles back into the tank.
If you’re looking for a simple vertical filter for your tank, this is a good small option for most little tanks. It doesn’t take up much space and looks discreet in the tank.
Here’s a more holistic look at it…
4 AquaTop Internal Filter
A filter meant for small to medium tanks, this filter is available for tanks up to 10, 20, 30, or 40 gallons. The flow rate of each filter matches the maximum tank rating, with the smallest starting at 42 GPH.
AquaTop’s internal filter doesn’t take up a lot of space. It’s also energy efficient, with the smallest model running on only 2.5W of power.
This is a sponge filter. Inside, there are no cartridges or other types of filtration media, only sponge filtration. You can use a smaller sponge and include biological media, if desired. However, the sponge may also act as a bio filter over time if used only with tank water.
The filter is fully submersible. Place it anywhere within the tank. Water returns to the tank through the spray bar, which can be submerged or above the water line for added aeration.
For simplicity, this is a great choice. Maintenance is easy, and the filter is simple even for beginners to set up and use.
Let’s take a look at it from both sides…
5 Fluval Underwater Filter
There are 4 different size models of the Fluval underwater filter. They are up to 15 gallons, 12-30 gallons, 24-40 gallons, and 34-65 gallons.
This is a fully submersible filter. Place it vertically or horizontally anywhere in the tank. Water sucks in through a bottom intake nozzle and returns to the tank from a multi-directional output valve.
Use cartridges for filtration in this Fluval. There is space for one or two cartridges, depending on the size of filter you get, and some loose media such as sponges and charcoal filtration. All initial filtration media is included in the kit when you purchase it.
Flow rate is adjustable for all size models. It attaches to the glass in the tank with built-in suction cups.
For a simple filtration option that works in a lot of different tank sizes, this is a nice choice. It’s got a varied filtration media setup to give you a little more customization. Consider this the submersible cousin to the popular Fluval 406 filter.
Here’s more on the full story…
6 Penn Plax Cascade 600
For tanks from 20 to 50 gallons in size, the Penn Plax Cascade 600 is a good choice. It’s a blue and black filter with a transparent media chamber.
This is a fully submersible filter that can be placed anywhere in the tank. Water comes in through an intake valve directly into the media chamber. The chamber is customizable, including for loose media.
You can choose to return the water to the tank through the bare output valve or a spray bar. Water can be redirected to lower the impact of the return current. Flow rate is also adjustable.
Use any type of media that fits into the chamber. Sponges and loose media of all types are compatible with this filter. Some filtration media is included to get you started.
When you want more options in filtration media without moving to external filtration, this is a nice option.
Is it all good news? Read on…
7 Tetra Whisper In-Tank Filter
The Tetra Whisper In-Tank Filter comes in 5 different size options: up to 3, 4, 10, 20, or 40 gallons. Each model has the same basic design with different sizes depending on the gallon rating.
Filtration media cartridges are laid out vertically for easy access. You can easily reach them to change cartridges or clean them. Cartridges are available for replacement.
This is a partially submersible filter. Do not submerge it fully underwater. Place it vertically with only the bottom half in the water. The entire filter fits neatly into the tank without any parts hanging externally.
For all kinds of tanks, nor just fish tanks, this filter gives you a little bit of flexibility. The addition of an air pump makes it nice for tanks that need more oxygen.
What else should you know?
8 MarineLand Magnum Polishing Internal Filter
You can purchase the MarineLand Magnum polishing internal filter in a pack of 1, 2, 3, or 4 filters at once. Each filter is the same size and works on tanks up to 97 gallons in size. The maximum flow rate is around 290 GPH for this filter.
As a canister, it has contained filtration media inside. The unique inner setup uses a series of sponges, media cartridges, and other parts to perform all 3 types of filtration. There are 2 media chambers that are customizable.
Because it’s fully submersible, no priming is necessary. This is a watertight unit that can be placed anywhere within the tank without worry.
An additional water polishing option makes water clearer. It removes fine dust particles and tiny debris from the water, leaving it much clearer and less foggy overall.
When you need a submersible filter in a large tank, you don’t have a lot of options. The MarineLand Magnum gives you a good option for medium to large aquariums instead of just smaller tanks.
Is it worth it? You decide…
What To Look For In An Internal Tank Filter
Internal filters sit underwater, entirely inside the tank. Because of this unique setup, there are a few extra things for you to pay attention to when you’re buying one. Here’s are the main things that matter for an internal filter:
- Flow Rate
Depending on the size of your tank, you’ll need a certain flow rate. Larger tanks need stronger flow rates while smaller tanks can get by with lower flow rates. It’s difficult to overpower a tank, though it is possible if you get a filter that’s rated for much larger tanks.
In general, the flow rate of the filter should be 3 to 4 times the number of gallons in the tank. Flow rates are measured in gallons per hour (GPH). This means that all the water in the tank can go through the filter at least 3 times per hour.
Internal filters are often small and have lower flow rates. This is fine, as long as you don’t try to use them for larger tanks. You can put multiple internal filters in the same tank to improve the flow rate.
Tanks that are fully stocked or overstocked need more filtration power. If you have messier fish, like goldfish, you also need a stronger filter.
- Fully Or Partially Submersible
Not all internal filters are fully submersible. Pay attention to this. It affects how you’ll install the filter and where it goes in the tank.
Partially submersible filters cannot go on the bottom of the tank, unless it’s a shallow turtle tank. Fully submersible filters go anywhere, but they should always be covered with water. Choose the internal filter type that best suits your tank.
- Filtration Media Types
Smaller filters may limit the types of media you can add inside the filter. You want a filter that can do mechanical, biological, and chemical filtration. Even small tanks benefit from this combination.
Most internal filters use cartridge filtration media. Cartridges are convenient, but they limit what you can use and how much of each type of media you can have. If it uses cartridges, make sure the available cartridges cover your needs.
- Filter Style
Not all internal filters look or operate the same way. Some stick onto the tank glass near the top of the water while others can lay horizontally on the bottom of the tank. Find a shape and style you like for your tank, small or large.
Remember that you have to clean and re-stock your filter to keep it working at its peak. Simple maintenance is a blessing. Get a filter that’s as easy to clean as possible.