Filter For A 20 Gallon Aquarium( Types, Price, Size )

A 20 Gallon fish tank is the perfect size for beginners or Kids bedrooms but without the correct filter, a 20 Gallon tank will soon become smelly, look terrible and have poor water conditions.

This is not an acceptable aquarium to keep pet fish in. As responsible fish keepers with have a duty of care to provide the best possible conditions for your fish to thrive in.

Without a suitable filter in your aquarium you will not enjoy the beautiful sight of a thriving home aquarium with healthy fish.

In this article: Filters For A 20 Gallon Aquarium. We’ll look at all the best possible filter options for your tank and narrow the search down to just 3 of the best type of filters for that size aquarium. There are hundreds of filters, makes and models on the market and choosing the best one for your aquarium can be confusing and often overwheleming

So, Let us help you make the right purchase and find the most suitable filter for your 20 Gallon ( 90 Litres ) Aquarium.

First, Lets look at the three main options on the market and the most commonly used filters. Then we’ll explain why these filters are the most popular and give you some Pros & Cons for each one. Finally, we will give you a link to the most suitable filter for your size aquarium and direct you to the cheapest place we found for that filter online. This will save you time and money.

The most common Type of 20 Gallon Aquarium filters are:

  • UnderGravel Filters
  • HOB ( Hang On The Back Filters )
  • Internal Power Filters

You have an extra option of using an external canister filter. However, these are best used for larger aquariums and not really neccassery for a smaller 20 Gallon Aquarium. In our article: Filters for a 30 Gallon Aquarium we touch on this type of filter more.

What are Undergravel filters?

An undergravel filter is a plastic plate/sheet that is placed in the bottom of your aquarium and then covered with gravel. The water is drawn up through the uplift tubes along with bubbles from an air pump and an air stone. This pulls water from under the plate, which in turn draws water down through the gravel trapping dirt within the gravel. Then, mother nature takes over and the friendly bacteria that live within the gravel breaks down the dirt and fish waste.

This is one of the oldest methods of filtering a 20 Gallon aquarium and is tried and tested and works well. The only issue you will face with this method is that it really needs to be installed when the tank is new. Adding one to an existing aquarium is hard!


  • Very effective
  • Inexpensive
  • Reliable and if the pump fails it’s easy to change
  • Natural way to filter an aquarium
  • Takes up no space in the aquarium
  • Out of sight


  • Cannot be added to an established aquarium without lost of mess and stress to the fish
  • Hard to clean
  • Gravel will become compacted and will need regular cleaning

What is a HOB filter?

A ( HOB ) Hang On The Back Filter is exactly what is says. A filter that hangs on the back of your aquarium.

The units can be added at any point which makes them suitable for most fish tanks.

This unit hangs on the side or back of your aquarium and has one of two downpipes that reach into your aquarium. With the help of a motor in the HOB unit water is drawn up the pipes which bring water and debris with it which is then passed through a series of filtration medias which physically and biologically capture and break down the fish waste and dirt.


  • Very easy to install
  • Easy to clean
  • You can use different types of media
  • Inexpensive
  • Various sizes available for all tanks


  • You need space at the back or side of the aquarium
  • The sound of running water can be annoying
  • Some are noisy
  • Can be difficult to prime and start going.

What is a Internal Power filter?

These filters are the most popular and have been for the past few years as technology has improved and made them one of the most effeciant methods of filtering aquarium water for any fish tank between 10-30 gallons.

They are plastic canisters that contain filtration media which traps the dirt and waste as it passes through the canister which is powered by a waterproof motor. They come in all different sizes and can be used in freshwater and saltwater aquariums.

It’s a very efficient and effective way to clean a fish tank for very little money. Easy to clean and easy to install makes them one of the first choices to consider when buying a filter for a 20 Gallon ( 90 Litre ) fish tank.


  • Easy to install into a new or established aquarium
  • You can use different types of filtration media
  • Inexpensive
  • Suitable for all size aquariums
  • Creates a lovely water flow within the tank which helps oxygenate the water


  • Take up space within the tank
  • Spoils the appearance of the aquarium
  • Get your hand wet when cleaning
  • Have been known to suck up fry

What is GPH?

GPS stands for Gallons Per Hour and is the amount of water that passes through the filter every hour in Gallons.

The best way to know that you have a suitable filter for the size of your aquarium is to buy on that filters 110-120% of your water every hour. This will ensure sufficient water circulation and the best possible filter for the volume of water in your aquarium.

As an example, the HOB filter we have chosen is a 30 GPH filter. This means that your entire aquarium water will pass through the filter at least once every hour.

If You had a 30 gallon aquarium then we would recommend a filter around 40-50 GPH. I think you get the idea.

Which filters are best for a 20 Gallon aquarium?

We have listed the three types of filters that we have mentioned above and found the three best models and price online. They have been selected for a number of reasons which include: Price, GPH , Style, Build Quality and performance.

Lee's 15/20 Original Undergravel Filter, 12-Inch by 24-Inch

 in stock
9 new from $14.81
as of July 23, 2024 3:30 pm


  • Made of special quality plastic which resists splitting and cracking
  • Also has optional flow-thru set-up for use with a power head
  • Size: 12-inch by 24-inch

Fluval C2 Power Filter

 in stock
4 new from $21.63
2 used from $14.65
Free shipping
as of July 23, 2024 3:30 pm


  • Ceramic heater core for even and efficient heat distribution
  • Filters 119 gallons per hour. Flow Rate: 120V/60Hz – 1,000 L/h (264 US Gal/h) and 230-240V/50Hz –900 L/h (238 US Gal/h)
  • Equipped with cleaning indicator for when the polyfoam needs to be rinsed
  • Clip on filter; Quick and easy maintenance
  • Helps provide a stress-free environment; Works to maintain refreshing crisp water

Fluval U2 Underwater Filter

$34.35  in stock
6 new from $34.35
1 used from $27.35
Free shipping
as of July 23, 2024 3:30 pm


  • Filters debris and provides optimal biological filtration
  • Ideal for fresh or saltwater aquariums. Wattage - 120V/60Hz – 5W and 230-240V/50Hz – 5W. Output - 105 US Gallon
  • Silent and efficient
  • Ideal supplemental or stand-alone filter, or for installations where external filtration is not possible
  • Comes with 3-way flow control with adjustable output, 2 foam pads, 2 poly/carbon cartridges and 40 grams BioMax; Ideal for 12-30 gallon environments

Conclusion : 20 Gallon Fish Tank Filter Review

No matter what size aquarium you own you MUST have a suitable filter to provide your tropical or saltwater fish with the cleanest water packed with oxygen for them to thrive.

Whichever one you choose above is fine by us. You can work out which is the right one for you very easily if you ask yourself a couple of questions. Then it’s a process of elimination and you’ll have the right filter for your situation in the end.

We hope you found this article helpful. If you have a different size aquarium then we have a review for most size tanks 10 Gallon, 30 Gallon and 50 Gallons plus. Go check them out!

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