Freshwater Aquarium Filters


This article will discuss the reason for aquarium filtration, the different types of aquarium filters, and provide some personal aquarium filter reviews about the filters that I currently use or have used in the past. Besides the size of your aquarium and the type of fish you want to keep, probably the next most important aspect of the aquarium hobby is what filter or combination of filters you will use. The type of fish you are going to stock the tank with will have a profound impact on what type of filtration you need and can have for this tank.

A tank of fancy goldfish is going to require substantial filtration as fancy goldfish are known to be massive waste producers where as if you are going to keep a lightly stocked tank of neon tetras, less filtration will be required.

Purpose of Aquarium Filtration

Aquarium filtration keeps water conditions optimal in a variety of ways through mechanically, biologically, and chemically filtering the water. A tank without any form of filtration will likely require more water changes and may not have enough surface area for beneficial bacteria to colonize. Filtration is also going to affect how much stock a tank can handle. (Check out the aquarium stocking calculator for a rough idea of whether your filtration will be sufficient)

To understand why filtration is so important, we need to first understand the nitrogen cycle and the effects each step in the cycle can have on our fish.

The Nitrogen Cycle

Photo of the nitrogen cycle in an aquarium
An illustration of the aquarium nitrogen cycle


Like all other animals, fish excrete waste in the form of ammonia. Ammonia levels need to be zero as any level of ammonia in the water is bad for your fish. Ammonia is the most common cause of death of fish in a new fish tank.  This is commonly referred to as "New Tank Syndrome".

What is New Tank Syndrome?

New Tank Syndrome occurs when fish are added to an un-cycled tank. An un-cycled tank is one where there isn't sufficient nitrifying bacteria to convert the ammonia to nitrites, the ammonia will accumulate in the tank water and cause ammonia poisoning. (A cycled tank is one where sufficient nitrifying bacteria has colonized the aquarium filter and tank to ensure ammonia and nitrites are broken down before they have a chance to harm the fish.)

What are the Signs of Ammonia Poisoning?

  • Fish gasping for air at the surface
  • Loss of appetite
  • Red streaks appearing on the body or gills
  • Inflamed gills
  • Lethargy (most notable in active fish)

If left untreated, ammonia poisoning will cause tissue damage and internal damage to the fish's internal organs.

What Actions to Take if There is Ammonia Poisoning?

If the ammonia levels are high in your tank, I recommend performing an immediate water change and adding a dose of "Seachem Prime" to your water. I personally use Seachem Prime and find that it is a great product. Not only does it remove chlorine and chloramine, but it can neutralize ammonia and nitrites.

The first part of the nitrogen cycle is the breakdown of ammonia into nitrites. This breakdown is performed by specific strains of nitrifying bacteria. These bacteria will slowly colonize the surfaces of your tank and filter.

How to Jump Start Cycling of a New Tank?

If you already have an established tank, place the filter for the new tank in this established tank. The filter will quickly colonize with beneficial bacteria, allowing you to speed up and potentially bypass the cycling process.

If you are new to the aquarium hobby and do not have any established tanks, you should ask a local aquarium shop for either a piece of substrate like a small rock or a small piece of their aquarium filtration media. This media or substrate medium will likely be colonized with the bacteria your tank needs to break down ammonia and nitrites. You'll want to place this media with the rest of your filtration so that the bacteria can colonize your new media. Make sure the media they are providing you stays wet, or the bacteria on it could die.

You should be testing your water for ammonia and nitrites daily for the first couple of weeks of the new tank and whenever you add more fish to the tank to ensure the filtration can handle the increased bio load.


Nitrites are the byproduct from the breakdown of Ammonia. Like ammonia, there are no safe levels of nitrites in your aquarium other than zero. Nitrites reduce a fish's ability to obtain oxygen and they will literally suffocate to death.

What are the Signs of Nitrite Poisoning?

  • Gills turning brown
  • Rapid gill movement
  • Fish gasp for breath at the water surface
  • Lethargy

Nitrites reduces the ability of the blood to carry oxygen by oxidizing hemoglobin to create methemoglobin. Methemoglobin doesn't have the ability to carry oxygen like hemoglobin. If your fish is exposed to nitrites long enough, they will literally suffocate from a lack of oxygen in their bodies. Like ammonia, you want to keep your nitrite levels at zero.

What Actions to Take if There is Nitrite Poisoning?

Perform an immediate water change to reduce the amount of nitrites in your water. Use Seachem Prime to neutralize the remaining nitrites in your water until there is sufficient bacteria in your tank to breakdown the nitrites. Per Seachem Prime's technical support, Prime will bind to ammonia, nitrite, and nitrate for 24 to 48 hours. So you'll want to maintain a steady dose every 24 to 48 hours until the nitrite levels are zero.

The second part of the nitrogen cycle is the conversion of nitrites to nitrates. The bacteria involved in this process will also colonize your tank and filtration media. This process can be sped up by following the steps in the "How to Jump Start Cycling of a New Tank?" section above.


Nitrates are not as harmful to aquarium fish, but they are still harmful in large concentrations. The nitrate levels that each fish can tolerate depends on the type of fish.

What are the Signs of Nitrate Poisoning

  • Fish experience a loss of equilibrium (you'll notice your fish swimming strangely)
  • Fish curl head to tail
  • Fish become listless
  • Exposure to high levels of nitrates can render fish sterile

How to Reduce Nitrates in Aquarium

  1. Water changes will help reduce nitrates in your tank
  2. Growing plants in the aquarium or hydroponically using land plants with roots in the aquarium water. Plants in the aquarium will use the nitrates as they grow
  3. Using filter media that has anaerobic areas to house denitrifying bacteria that can break down nitrates into nitrogen gas that will escape from the water surface

Now that we understand the different parts of the nitrogen cycle, let's review the different types of aquarium filtration available and the pros and cons of each aquarium filter.

Three forms of aquarium filtration

Various types of aquarium filter media: Mechanical filtration sponges in top left; ceramic type media for biological filtration in bottom left; examples of chemical filtration media on the right side


Mechanical filtration filters the aquarium water for fish waste and any other physical debris in the water. It is what keeps the water looking clear. Mechanical filtration is important because fish waste generates ammonia. By filtering the fish waste, you can clean the filter during water changes and that will get rid the aquarium system of large sources of ammonia.


Biological filtration refers to the bacteria that will breakdown the chemicals in the nitrogen cycle. This is important as any levels of ammonia and nitrites can cause stress and potentially death to your fish if left untreated. Biological filtration media tends to have a high surface area to volume ratio to provide the maximum amount of space for bacteria to colonize.


Chemical filtration uses various forms of media to remove chemical impurities from the water. The most common type of chemical filtration is activated carbon. Activated carbon removes organic compounds from the water that isn't normally handled by biological filtration. Another common type of chemical filtration is "Seachem Purigen" which also removes organic compounds from the water but also helps control ammonia, nitrites, and nitrates, and 'polishes' the water. When I used Seachem Purigen in my amazon tank, it removed the tannins that stained the water and I've never had aquarium water that was as clear as with purigen.

Image of hatchetfish in a planted tank
Hatchetfish relaxing in a planted tank

Different types of aquarium filters

Sponge Filters

Type of filtration

  • Mechanical
  • Biological

Sponge filters are great filters that provide mechanical and biological filtration up to the nitrate stage.


  • Cost effective
  • Great mechanical and biological filtration up to the nitrate stage
  • Great for fry tanks and fresh water shrimp tanks


  • Takes up space in the tank
  • Some aquarist consider them an eye sore
  • Can be loud if run with an air pump

Sponge filters are a great alternative for smaller tanks because of their low price, ease of use, and performance. They can be just as effective in a large tank and many aquarists still use sponge filters as their sole form of filtration.

The filters need to be attached to either an air pump or a powerhead. The air or powerhead will cause water to be drawn through the sponge, filtering out fish waste. The sponge will also provide plenty of surface area for beneficial nitrifying bacteria to grow. When rinsing the sponge, use de-chlorinated water as chlorine will kill the beneficial bacteria.

Hang on back filters (also known as power filters)

Type of filtration

  • Mechanical
  • Biological
  • Chemical

Hang on back (HOB) filters are another good low to mid cost filtration option.


  • Allows for a selection of different filter media depending on the tank needs
  • Easy to set up
  • Provides surface agitation for gas exchange in the water
  • Easily obtainable


  • Might create too strong a flow
  • Can be pretty loud compared to other types of filters
  • Might hold less media than a canister filter (need to compare the HOB to the canister filter you are considering)

HOB filters are incredibly versatile for their price. HOB filters are substantially lower cost than canister filters but provide the same benefit in terms of filter media. The one drawback I found when using the filters was that the sound of splashing water was pretty loud and the current was too strong for my guppies. As the filters get older, they tend to get louder due to vibrations. Other than that, if you have fish that love strong currents, an HOB is a great bet. Based on my own research and experience with Aquaclear filters, they are my preferred power filter. I've used am Aquaclear 20 all the way up to Aquaclear 110 filters and would highly recommend them.

I have not used any other brand of HOB filters because based on my research, Aquaclear is the preferred HOB filter compared to the other brands.

Image of a Fluval 306 aquarium filter
Fluval 306

Canister Filters

Type of filtration

  • Mechanical
  • Biological
  • Chemical

Out of all the filters I have used, canister filters are my favorite. They also happen to be the most expensive filters out there as well, but they can represent good value depending on your needs.


  • Most aquarium canister filters are pretty straightforward to set up
  • They allow for a variety of filter media depending on the tank needs
  • Quiet (my Fluval FX6 is so quiet, I've checked it multiple times to make sure it's still operating)
  • Aesthetically pleasing (stays out of sight and some of them provide minimal intrusion into your tank)


  • Might create too strong a flow
  • Depending on the type you get, they could be pretty heavy which makes it a pain when you're cleaning it
  • Expensive

If you are just starting out, a canister filter is a costly investment and you may want to try a sponge or HOB filter first to ensure you like the hobby. If you are seasoned and have a decent size aquarium tank, I wouldn't hesitate to recommend a canister filter. For those of you who like aesthetics, the great thing about the filter is it remains out of sight. For those who want to ensure there is adequate filtration for your tank, canister filters can hold the most media out of any of these other filters unless you create your own sump filter. Even the Aquaclear 150 HOB can only hold so much media after you have the sponge in there.

I currently own a Fluval FX6 on my 70 gallon tank. The FX6 is the best canister filter I have ever used. It's quiet, I can fill it with so much great filtration media, and my water parameters have never been better. Despite being the best canister filter in my opinion, it is also one of the most expensive and at the end of the day, it isn't suitable for any tank smaller than a 75 gallon. Check out our recommendations for aquarium external canister filters in 2017 in our blog post.

Undergravel filter

Type of filtration

  • Biological

Undergravel filters are still used today, although I have personally never used one.


  • Stays out of sight under the gravel
  • Biological filtration


  • Only good for biological filtration (uses gravel to colonize beneficial bacteria)
  • Required to be weighed down with gravel which can provide pockets for debris to collect in
  • Potential issues with gas build up in the gravel that could be harmful to the aquarium if released

I have no experience with this type of filter. In my opinion, I would much prefer to choose one of the three filters listed above.

Internal filters

Type of filtration

  • Mechanical
  • Biological
  • Chemical

Internal filters are a canister filter that operate within the aquarium.


  • Cheaper than canister filter
  • Allows for mix of filtration media


  • Can't hold as much media as canister filters and some HOB filters
  • Takes up tank space
  • Expensive

I've never used an internal filter. I have considered one, as they are cheaper than the canister filters but I'm sure glad I didn't get one. I just don't feel the value for the price is there and if you're willing to pay the price of admission, why not get a better external canister filter. I just can't see the niche that this product is trying to attract.

Filter media


There are different types of filter media for the different types of aquarium filtration. Refer to the attached image for some examples of mechanical, biological, and chemical filtration.

Various types of aquarium filter media: Mechanical filtration sponges in top left; ceramic type media for biological filtration in bottom left; examples of chemical filtration media on the right side

The importance of filtration and the different types of filtration

Aquarium filtration media remove organic and inorganic compounds and waste from the water. One of the main reasons filtration is required in a tank is due to the nitrogen cycle and how if the compounds in the nitrogen cycle aren't removed, they can damage or kill the inhabitants of your aquarium.

Nitrogen cycle

Photo of the nitrogen cycle in an aquarium
An illustration of the aquarium nitrogen cycle

Please refer to the explanation of the nitrogen cycle earlier on in this aquarium filters page for a detailed explanation of the nitrogen cycle. Summary of the nitrogen cycle: fish waste produces ammonia. Ammonia is toxic to the fish and the bacteria in your filter will break down that ammonia into Nitrites. Again Nitrites are toxic to the fish and require a different strain of bacteria to break it down into Nitrates which can be removed from the water with plants, or special bacteria, or by water changes.

Ammonia ----> Nitrites ----> Nitrates ----> Water changes/plants/bacteria

Mechanical media

Various sponge media that can be used in your power or canister filter

The purpose of mechanical filtration is to remove the physical fish waste from your water. It plays an important role with water clarity and most mechanical filtration will provide surfaces for bacteria to colonize. This means that mechanical media will help with the first couple steps of the nitrogen cycle.

The above is the reason why smaller tanks and shrimp tanks can have success with just a sponge filter as it will provide sufficient biological filtration to ensure ammonia gets converted to nitrate. It will not however, be able to provide the anaerobic areas required for de-nitrifying bacteria to convert nitrates to nitrogen gas. For this, you'll need to look at special filter biological filter media.

Examples of biological aquarium filtration media

Biological media

The purpose of biological media is to colonize the beneficial bacteria necessary to breakdown fish waste and the various waste components of the nitrogen cycle. Because biological media is typically measured in terms of surface area, the most popular types of media are ceramic media that are specially created with cracks to increase the surface area.

One benefit that these types of media have is that they are able to host de-nitrifying bacteria in these cracks as they are anaerobic bacteria that need water devoid of oxygen to operate. The theory behind why denitrifying bacteria can survive in these nooks and crannies is:

1) Regular nitrifying bacteria found on the surface and at the begin of a biological filter will use up oxygen as it breaks down ammonia into nitrites and nitrites into nitrates

2) As a result, the water should lose oxygen as it moves through the filter

3) This de-oxygenated water will then reach the de-nitrifying bacteria in these cracks and grooves of the ceramic media and allow for removal of some of the nitrates from the water

Unless your aquarium fish tank is heavily planted and has a low population of fish and critters in it, it is near impossible for nitrates to be reduced to 0 even with massive amounts of ceramic biological media. What the media will allow you to do though is to reduce water change frequency which should allow you to spend more of your time enjoying your tank instead of maintaining it.

Chemical media

Examples of chemical filtration. Ammonia absorber on the left and activated carbon on the right.

Activated Carbon

Activated carbon is probably the most common chemical filter media which uses charcoal or activated carbon to absorb organic and inorganic materials dissolved in the tank water. So how does activated carbon work? It removes impurities and materials in the water through two processes, adsorption and absorption.


Due to the many pores in carbon, the large surface area provides multiple chemical bonding sites for chemicals in the water to bind to. This process is call Adsorption. This process also explains the reason that carbon needs to be changed on a regular basis as these chemical bonding sites will be used up over time.


The process of absorption occurs by trapping minor compounds in its many tiny pores. This can be considered to be a form of ultra fine mechanical filtration.

Uses of activated carbon

  • Absorbs odors - If you have any funky smells coming from your fish tank and can't figure out where its coming from, activated carbon can absorb the phenols causing the smells
  • Removes tannins - Whenever you put a piece of driftwood or leaf matter into your tank, you might notice that the water will get a brownish tinge. This is due to the tannins that are being released by the driftwood or leaf matter. Tannins are beneficial to the inhabitants in your tank but if you want clear water, activated carbon can absorb the tannins
  • Remove chlorine and chloramines - I recommend using a product that mixes with water before you pour it into your tank, but if you are concerned that you may not have used enough of the product, activated carbon can put your mind at ease
  • Removal of medicinal products - Once you have completed treatment of a tank, activated carbon can be used to remove the medication from the water

Downside of activated carbon

Activated carbon often contains some amounts of phosphates which often leads to algae outbreaks in the tank. It is important not to overuse activated carbon in a tank.


There have been rumors that if you do not change activated carbon on a regular basis, the compounds it has absorbed will be released (de-absorbed) back into the water over time. This is not an accurate claim and the water conditions in an aquarium will not provide the conditions for such an event to occur.


Activated carbon can be useful for specific applications in a freshwater aquarium. At the end of the day, there is nothing activated carbon provides that cannot be done with regular water changes. That being said, for aesthetic purposes, the removal of tannins and odors will make your aquarium more pleasant for all viewing and using it.

Synthetic Polymers

Synthetic Polymers are similar to activated carbon except they can absorb ammonia, nitrites, and nitrates and can be recharged and reused over multiple times.

Currently the most popular synthetic polymer is Seachem Purigen. Purigen boasts the ability to make your water crystal clear (like carbon), but also provides control for ammonia, nitrites, and nitrogen which is something activated carbon does not do. Lastly, Purigen can be recharged using bleach and water. Please be sure to rinse the product very well after recharging, as any bleach is going to do a lot of harm to your tanks inhabitants.


There are many in the freshwater aquarium community that swear by Seachem Purigen and use it religiously in their tanks. For peace of mind and crystal clear water, it is definitely not a bad media to have for your tanks filtration. Bear in mind, it needs to be removed if you are going to be medicating the tank and when it darkens, it will need to be recharged or replaced. I have used the product in one of my tanks and true to its word the water was crystal clear, removing all the tannins in the tank. I haven't used it since though as I think tannins are beneficial and I like the tea color for my rummy nose tetras.