Your aquarium filter might be the most important piece of equipment you buy for your fish tank.
Unfortunately, choosing one can be confusing.
Hang-on-back filters, canister filters, sponge filters, internal filters, sumps, filter cartridges, ceramic media, activated carbon. If you are setting up your first aquarium, it can feel like you need to learn an entirely new language just to keep the water clean.
The good news is that aquarium filtration is much easier to understand once you know what a filter is actually supposed to accomplish.
In this guide, we’ll explain the different types of aquarium filtration, compare the most common filters, and help you determine which option makes sense for your tank.
It is easy to think of an aquarium filter as something that simply removes dirt from the water.
That is only part of its job.
A good aquarium filter can provide three different types of filtration:
Not every aquarium needs all three at all times, but understanding them will make choosing and maintaining your filter much easier.
Mechanical filtration removes physical debris from aquarium water.
Water passes through material such as:
These materials capture particles such as leftover food, fish waste, plant debris, and other material floating through the aquarium.
Think of mechanical filtration like a strainer.
Water passes through while larger particles become trapped in the media.
Eventually, mechanical media becomes dirty and needs to be rinsed or replaced depending on the type of material being used.
Mechanical filtration helps keep the aquarium looking clean, but it is not the only reason your filter matters.
Biological filtration is arguably the most important function of an aquarium filter.
Fish produce waste, and that waste eventually contributes to ammonia in the aquarium.
Ammonia is toxic to fish.
Beneficial bacteria process ammonia into nitrite, which is also toxic. Another bacterial population then processes nitrite into the much less toxic nitrate.
This is the nitrogen cycle.
A large portion of these beneficial bacteria live on surfaces inside your aquarium, particularly within your filter media.
Biological filter media is designed to provide lots of surface area for these bacteria to colonize.
Common examples include:
This is why you should be careful when cleaning your aquarium filter.
That dirty-looking sponge may contain part of the bacterial population keeping your fish alive.
Chemical filtration uses specialized media to remove certain substances from aquarium water.
Activated carbon is probably the most familiar example.
Depending on the media, chemical filtration can be used to help remove:
Other specialized media can target substances such as phosphate or other compounds.
Chemical filtration can be useful, but it is not always necessary to run every type of chemical media continuously.
The appropriate media depends on what you are trying to accomplish.
Most home aquariums use one of several common filtration systems.
The best option depends on your aquarium size, livestock, available space, maintenance preferences, and budget.
Let’s look at the major choices.
A hang-on-back filter, commonly called a HOB filter, hangs on the rim of the aquarium.
A small intake tube pulls water from the tank into the filter. The water passes through filter media and then flows back into the aquarium.
For many beginner freshwater aquariums, a quality hang-on-back filter is an excellent choice.
They are simple, effective, and easy to maintain.
A sponge filter is one of the simplest aquarium filters available.
An air pump pushes air through the filter, creating water movement through a sponge.
The sponge captures debris while also providing a large surface for beneficial bacteria.
Sponge filters are particularly useful in tanks containing delicate animals.
A standard filter intake can potentially suck up tiny shrimp or newly hatched fish. Sponge filters eliminate much of that concern.
Their simplicity also makes them extremely reliable.
Canister filters are external filters commonly placed underneath or beside the aquarium.
Water travels from the aquarium through tubing into a sealed canister containing several layers of filter media. A pump then returns the filtered water to the tank.
Canister filters are popular for medium and large freshwater aquariums because they can hold significantly more media than many smaller filters.
A canister filter can be a great choice for a larger freshwater aquarium, heavily stocked community tank, or aquascape where you want most equipment hidden from view.
Internal aquarium filters sit completely or partially inside the aquarium.
They use an electric pump to pull water through filter media before returning it directly to the tank.
Internal filters can be useful when space outside the aquarium is limited or when you want a straightforward all-in-one filtration device.
All-in-one aquariums, often abbreviated as AIO aquariums, contain a filtration compartment built directly into the aquarium.
Water usually flows through openings into a hidden rear or side chamber where filter media, pumps, heaters, and other equipment can be placed.
AIO systems are particularly appealing if you want a clean, modern aquarium without visible equipment hanging around the display.
They are also extremely popular for smaller reef aquariums.
A sump is a separate container, usually located underneath the main aquarium.
Water drains from the display aquarium into the sump, passes through filtration and equipment, and is then pumped back into the display.
Sumps are particularly common on larger saltwater and reef aquariums.
Sumps are extremely versatile, but they are not necessary for every aquarium.
A beginner setting up a straightforward 20-gallon freshwater tank probably does not need one.
Someone building a 150-gallon reef aquarium may find a sump extremely useful.
Undergravel filters were once extremely common in freshwater aquariums.
A perforated plate sits underneath the gravel, and water is pulled through the substrate.
The gravel itself becomes part of the biological filtration system.
They can work, but modern aquarium keepers have many other filtration options that are easier to maintain and customize.
For most new aquarium setups, a HOB, sponge, internal, or canister filter will usually be a more practical choice.
For a typical beginner freshwater aquarium, we generally recommend either:
A hang-on-back filter for a simple community aquarium
or
A sponge filter for smaller tanks, shrimp tanks, breeding setups, or aquariums containing delicate fish.
A HOB filter provides a nice combination of mechanical and biological filtration while remaining easy to access and maintain.
Sponge filters are inexpensive, reliable, and excellent for biological filtration.
Neither requires complicated plumbing or extensive aquarium experience.
As aquarium size increases, canister filters become particularly attractive.
Their larger media capacity allows you to use multiple types of mechanical and biological media while keeping most of the equipment outside the display.
Some large freshwater aquariums use more than one filter.
For example, a large aquarium might use two canister filters or combine a canister filter with a sponge filter.
Using multiple filters can also provide redundancy if one filter stops operating.
There is no single correct answer.
A smaller saltwater aquarium may use an AIO filtration chamber or hang-on-back system.
Larger reef aquariums frequently use sumps.
Saltwater aquariums may also incorporate additional equipment such as:
It is important to remember that a protein skimmer is not a replacement for biological filtration.
Marine aquariums still rely heavily on beneficial bacteria to process waste.
Aquarium filters are usually rated for a certain aquarium volume.
You might see a filter advertised as suitable for tanks up to 20 gallons, 50 gallons, or 100 gallons.
Treat these ratings as a starting point rather than an absolute guarantee.
The filtration needs of two 40-gallon aquariums can be very different.
A lightly stocked planted aquarium may produce relatively little waste.
A heavily stocked aquarium containing large, messy fish may require significantly more filtration.
When choosing a filter, consider:
Having some additional filtration capacity is generally preferable to choosing a filter that barely meets your aquarium’s needs.
When shopping for aquarium filters and pumps, you will frequently see the abbreviation GPH.
GPH means gallons per hour.
A filter rated at 200 GPH can theoretically move approximately 200 gallons of water per hour under its specified conditions.
This number helps you understand how quickly water circulates through the filtration system.
However, advertised flow rates do not always represent the exact flow you will see after adding filter media, tubing, equipment, and normal resistance.
Flow is also not the same thing as filtration quality.
A filter moving huge amounts of water through very little biological media is not automatically better than a slower filter with excellent media capacity.
Yes.
More flow is not always better.
Different fish come from different environments.
Some species appreciate strong currents and highly oxygenated water. Others naturally inhabit slower-moving environments and can become stressed if they spend all day fighting against the filter output.
Bettas are a common example of fish that often prefer gentler water movement.
Fish with long fins, tiny fish, shrimp, and fry may also struggle with extremely strong flow.
Choose filtration based on the animals living in the aquarium, not simply the largest number printed on the box.
Filter media is the material inside your filter that actually performs much of the filtration.
Rather than thinking of your filter as one object, it can help to think of it as a container that moves water through different types of media.
A common arrangement might include:
First stage: Coarse sponge for larger debris
Second stage: Fine mechanical media for smaller particles
Third stage: Biological media for beneficial bacteria
Optional stage: Chemical media such as activated carbon
The exact arrangement depends on your filter and aquarium.
Canister filters and AIO systems often provide considerably more freedom to customize media than disposable cartridge-based filters.
Not always.
Activated carbon can be useful for removing certain dissolved compounds, odors, discoloration, and residual medications.
However, an aquarium does not automatically require fresh activated carbon running every day.
Many successful aquariums operate primarily with mechanical and biological filtration.
Chemical media is best chosen based on a specific need rather than automatically added because there is an empty compartment in the filter.
This is where beginners can accidentally create problems.
Many replacement filter cartridges contain both mechanical media and some of the surfaces colonized by beneficial bacteria.
Throwing away the entire cartridge and replacing it with a sterile new one can remove part of your biological filtration.
The better approach depends on the design of your filter.
Whenever possible, maintain permanent biological media in the filter that does not need frequent replacement.
Mechanical media can be rinsed or replaced when appropriate without removing every piece of established biological media at the same time.
Your filter should be cleaned periodically, but it does not need to be sterilized.
In fact, sterilizing it is exactly what you want to avoid.
During a water change, you can use some of the removed aquarium water to gently rinse reusable sponges and biological media when they become clogged with debris.
The goal is to restore water flow while preserving the beneficial bacteria living on the media.
Avoid aggressively scrubbing every surface until it looks brand new.
An aquarium filter is supposed to contain microorganisms.
That is part of what makes it work.
Beneficial bacteria require oxygenated water.
When a filter is switched off, water stops moving through the biological media.
Short interruptions during maintenance are normally fine, but leaving a filter off for extended periods can begin affecting the bacterial community inside it.
Under normal circumstances, your aquarium filter should operate 24 hours a day, seven days a week.
You do not need to turn it off at night.
Your fish sleep.
Your filter does not.
If you notice that your filter has stopped, investigate it as soon as possible.
Check for:
How urgent the situation becomes depends on your aquarium, stocking level, oxygenation, and how long the filter remains offline.
Having an air pump and air stone available can be useful during equipment failures because it can help maintain surface agitation and oxygenation while you address the problem.
No.
This is one of the most important things for beginners to understand.
A filter processes waste and captures debris, but it does not make routine aquarium maintenance unnecessary.
Nitrate and other dissolved substances can gradually accumulate in the water.
Regular water changes help dilute those compounds and replenish the aquarium with fresh water.
Think of filtration and water changes as two parts of the same maintenance system.
You need both.
Live plants can certainly help.
Plants consume nutrients, including nitrogen compounds, as they grow. A heavily planted aquarium may process a surprising amount of waste.
But for most beginner aquariums, plants should complement filtration rather than replace it.
A traditional filter also provides water circulation, mechanical filtration, oxygenation through surface movement, and a reliable home for beneficial bacteria.
For most beginners, the answer can be kept fairly simple.
Small freshwater aquarium: Sponge filter, internal filter, or appropriately sized HOB filter
Typical 20 to 40-gallon community tank: Hang-on-back filter
Larger freshwater aquarium: Canister filter or multiple appropriately sized filters
Shrimp or breeding tank: Sponge filter
Clean, equipment-hidden setup: All-in-one aquarium or canister filter
Small saltwater aquarium: AIO filtration or an appropriate external filtration system
Large reef aquarium: Sump-based filtration is often worth considering
There are exceptions to every one of these recommendations.
The important thing is matching your filtration system to the aquarium rather than assuming there is one “best” filter for every tank.
Aquarium filtration is not just about making the water look clean.
Inside that filter is an entire biological community working constantly to process the waste produced by your fish.
Choose a filter with enough capacity for your aquarium. Give beneficial bacteria plenty of appropriate media to colonize. Maintain the filter without sterilizing it. Keep it running continuously.
Most importantly, remember that filtration is part of a larger system.
A good filter cannot compensate for severe overstocking, constant overfeeding, incompatible fish, or neglected water changes.
Combine appropriate filtration with a properly cycled aquarium, responsible stocking, regular water testing, and routine maintenance, and you will have a much stronger foundation for a healthy aquarium.
