In this article, I’ll review the top 5 filters suitable for 20 gallon tanks and reveal the number one pick so you can get the best filter for your aquarium.
Quick Overview: Top Pick
The best filter overall for 20 gallon tanks is the Seachem Tidal filter.
But, if you want to keep shrimp, you should look at getting the AquaClear 30 and add a sponge prefilter so your buds don’t get sucked up.
Why is an Aquarium Filter Important?
The Nitrogen Cycle
The aquarium nitrogen cycle is the natural process that breaks down toxic waste products in the water and transforms them into safe compounds.
Without this process, keeping fish in an aquarium would be impossible, so let me explain for anyone who’s not familiar with it.
Fish constantly put off waste, both urine and feces.
Gross, but true.
All that waste sinks to the bottom of the tank and starts to decompose.
As it breaks down, it puts off ammonia (NH3). This is bad news because ammonia is highly toxic.
Left on its own, the tank would quickly become unlivable. The fish would keep putting off waste which would make more and more ammonia until the water became completely toxic and all the fish died.
This is where the nitrogen cycle comes in. In an established tank, there are millions of beneficial bacteria living in the filter.
One kind eats ammonia and turns it into something called nitrite (NO2 -1), which is just as toxic as ammonia. Luckily, another kind of bacteria eats it almost instantly and turns it into a compound called nitrate (NO3-).
Nitrate is much less toxic and can be allowed to build up in the water column in between water changes.
The beneficial bacteria that carry out the nitrogen cycle primarily live in an aquarium’s filter. So a filter is vital because it helps process waste and make the water safe for fish.
Read More: The Nitrogen Cycle
Biomedia
The beneficial bacteria that carry out the nitrogen cycle don’t just float around in the water. They need a surface to cling to so they can pull waste out of the water column.
The walls of the filter housing just do not have enough surface area for the huge population of bacteria you need to run your tank.
That’s where biomedia comes in. It’s special filter media made out of a porous material (unglazed ceramic, sintered glass, etc) that water can flow through.
Biomedia increases the surface area inside your filter so there’s a lot more space for beneficial bacteria to set up shop.
It’s important to maximize biomedia in your filter. The more your tank has, the more easily the bacteria can process the waste produced by fish.
Why I Don’t Recommend Filter Cartridges
Filter manufacturers try to make it seem like their cartridges are some innovative invention that will magically destroy all impurities in your water.
In reality, a filter cartridge is really just some activated carbon sandwiched in between two pieces of filter floss.
That’s it. No magical properties at all.
And usually, when a filter relies on cartridges, the space inside the filter housing is very limited, there’s no room to add extra biomedia.
Worse yet, the carbon in a cartridge wears out after a few weeks and needs to be replaced, which means throwing away the whole cartridge. So any beneficial bacteria that have grown on the cartridge get thrown away!
I recommend going with a filter that doesn’t use cartridges and instead buying one that has room for permanent biomedia.
GPH and Why It’s Important
GPH stands for “gallons per hour.” It’s the measure of how much water the motor in a filter is able to pump in an hour.
It’s generally recommended that your filter has a GPH equal to four times the volume of your tank. So, for a 20 gallon (76 liter) tank, you would want a filter with a GPH of at least 80.
However, I really do recommend that you go with a higher GPH whenever possible.
The more water gets pumped through the filter, the more efficiently wastes can be processed.
20 Gallon Aquarium Filters Reviewed
Pro Tip: Internal motor vs. external motor: this refers to two different styles of hang-on-the-back filters. Internal motors hang inside the main part of the aquarium. External motors are in the filter housing that hangs on the back of the tank.
1. Seachem Tidal 35 (Best Overall)
Key Features:
- GPH: 130
- Internal motor
- Surface skimmer
- Large filter media area
I’m really impressed with this Tidal filter made by Seachem. This filter has a ton of room for media. The large filter housing comes with its own reusable mechanical filter media. The rest of the space can be filled with whatever chemical and/or biomedia you want.
This filter features an internal motor that hangs inside the main aquarium. There are many benefits to this design.
Filters with internal motors don’t need to be primed (filling up the back of the filter with water) so they’re really easy to get running.
No worries that the filter won’t start up on its own after a power failure or a water change. It will immediately start to suction water as soon as it has power.
Internal motors are also very quiet because the water absorbs any sound the motor makes.
As much as I like this filter, there are some downsides.
The biggest for me is that you can’t make this filter safe for tiny inhabitants, like ornamental shrimp or fish fry.
Since there’s no lift tube, there isn’t a way to attach a sponge pre-filter to keep little guys from getting sucked into the filter, where they can be injured and/or killed.
Also, you can’t shut off or cover up the surface skimmer. So again, tiny critters will end up being sucked into the filter and possibly dying.
I also don’t like that the surface skimmer can accidentally suck up fish food that gets too close to the filter. Uneaten food rotting inside the filter can foul your water.
I honestly wish there was a way to bypass or turn the skimmer off. It’s unnecessary in my opinion.
Pros:
- Quiet operation
- No priming
- Large biomedia capacity
Cons:
- Not safe for shrimp or fry
- Surface skimmer can suck up food
2. AquaClear 30 (best for Shrimp)
Key Features:
- GPH: 150
- External motor
I really love AquaClear filters. I’m currently running four of them in my personal tanks right now. I’ve been using them for years and have some real insight into this brand.
I like that you get a large open space in the filter housing that lets you add lots of biomedia. I have mine packed with lots of unglazed ceramic in a mesh bag.
It really works well.
It also comes with a large reusable sponge for mechanical filtration. The amount of gunk that little sponge can trap is unbelievable.
Also, if you want to keep shrimp or fry, you can easily fit this filter with a sponge pre-filter that will block them from getting sucked in.
AquaClears are really reliable long term, but they can be touchy to get started sometimes. Since it has an external motor, you do have to prime this filter.
And if there’s any kind of power outage, even for only a few minutes, you have to check to make sure that the filter has started back up on its own.
If it’s being touchy, cleaning out the impeller and the impeller well is usually all that’s needed to get it back up and running.
But, I’ll admit, it can be really annoying to have to do this when you’re trying to get your filter started after spending over an hour doing a water change.
But overall, these filters are little workhorses that definitely do an excellent job.
Pros:
- Large biomedia capacity
- Easy to make shrimp/fry safe
Cons:
- Needs to be primed
- Can be a pain to get started
3. Fluval C Power Filter
Key Features:
- GPH: 119
- External motor
- Upright mechanical filter media
- Easy to remove compartments for various media
I can sum up the Fluval C in a single word: fanciness.
This thing looks like the fish filter of the future.
But seriously, Fluval has put a ton of thoughtful design into this filter.
Every stage of filter media has its own compartment and everything fits together in a way to maximize flow through each stage.
I don’t like how small the compartment for the biomedia is. However, since I typically don’t run activated carbon in my filters, I would just stuff the carbon compartment with more biomedia.
I do like the vertical mechanical media and how easy it is to access. The mechanical media is usually the thing you need to clean out the most frequently so being able to just grab it out of there is awesome in my book.
Also, I have a lot of faith in the quality and reliability of Fluval products. They’ve really proven themselves to be leaders in the industry over the years.
Pros:
- Easy access to media
- Innovative design
Cons:
- Small biomedia compartment
4. Fluval 107 Performance Canister Filter
Key Features:
- GPH: 145
- Quick release hoses
- Ergonomic design for easy priming
- Central handle for media trays
All right, so this is a different style of filter than the others I’ve covered so far.
Canister filters offer a huge biomedia capacity and are the best choice if you plan on heavily stocking your tank with fish.
The basic design of a canister gives you an enormous amount of space for filter media. You can easily add around five times as much media in a canister than you can a hang-on-the-back.
Just like the Fluval hang-on-the-back filter, the 107 features vertical mechanical filtration that can easily be removed for cleaning.
Fluval has added several other design features centered around making maintenance easier.
There is a handle that lets you pull all of the media trays out at once, instead of having to dig around in a canister full of dirty water to pull out the bottom trays.
Also, the priming mechanism features an ergonomic finger loop that lets you easily work a small pump up and down to belch air out of the system to restart the filter after maintenance.
But, if you decide to go with a canister filter, any canister filter, instead of a hang-on-the-back, just be aware that filter maintenance does take a bit more work.
Pros:
- Unique priming handle
- Huge biomedia capacity
- Central handle for media removal
Cons:
- Canister filters require more involved maintenance
5. Aquarium Technology Hydro-Sponge Filter II
Key Features:
- GPH: variable
- Simplistic design
- Mechanical and biological filtration
Sponge filters are simple devices. Water is pumped through the large sponge, by either a powerhead or air pump. This gentle suction catches fish poop and other detritus.
The porous sponge material is the perfect media to grow beneficial bacteria that carry out the nitrogen cycle, which means a sponge filter provides both mechanical and biological filtration.
One thing I really love is that it’s so easy to set up and maintain one of these. It takes only minutes to put this together and just squeeze out the sponge several times in dechlorinated water to clean it. That’s it!
An air driven sponge filter is the safest thing to run in a fry or shrimp tank, in my opinion. There is nothing for tiny critters to get sucked into and many species will feed off of the layer of biofilm and gunk that grows on the surface of the sponge.
However, there is the downside that a sponge filter sitting in the tank is not exactly attractive looking and does take up some floor space.
But, you can easily disguise it by putting some tall plants and/or decor in front of it.
Also, this type of filter only provides mechanical and biological filtration, there’s no way to add on a chemical stage.
Pros:
- Simple to install and maintain
- Shrimp and fry safe
Cons:
- Bit of an eyesore
Pro Tip: Air driven sponge filters can be annoyingly loud. An easy fix for this is to add a small air diffuser so the stream of bubbles coming out of the top of the filter is a cloud of small bubbles instead of a gurgling string of large ones.
Which One is the Best?
That’s a hard question to answer. It all depends on your situation.
For a simple community tank, I’d probably go with the Seachem Tidal. I love filters with internal motors and this model has such a great biomedia capacity.
But, if you want to keep shrimp, the Tidal filter isn’t a good choice. However, it’s fairly simple to put a sponge prefilter on the intake of an AquaClear 30. That would keep them out of harm’s way.
If your tank will have tiny inhabitants, like teensy fish fry, I’d honestly go with an air driven sponge filter.
It gives you spectacular biofiltration without the risk of anyone getting sucked into a dangerous filter motor. Plus, the low flow is perfect for slow swimmers.
