Do bettas need a filter?

The simple answer to this question, and the correct answer in the vast majority of cases, is a simple ‘Yes’. But there’s a lot more to it than that, as we explain in detail below. Filtration is one of the most important aspects of betta keeping to educate yourself on, so dive right in and ask in the comments if you have any questions.


In this article we’ll be taking an in-depth look at betta filters, and answering some of the common questions that arise around the topic. Filtration is an important aspect of betta keeping, so it’s useful to have a basic understanding of how it works, and how you can craft a comfortable setup for your fish.

We’ve also written an article called ‘The Best Filter for a Betta’, and another about creating a baffle for you filter. If you are choosing a new filter for your betta, we recommend reading all three of our filtration articles.

We’ll get started by defining the terminology used throughout the article, then we’ll go on to answer some common questions about betta filtration.


Before we get started, we’ll reference some of the terminology that we use throughout the article so as to avoid confusion:

By our definition, an insert is a single medium, whereas a cartridge is two mediums combined.

This is illustrated in the diagram below, where the biological media is separate (and therefore regarded as an insert), whereas the chemical (carbon) and mechanical (sponge) are combined, and therefore make up a cartridge. We explain this diagram in more detail later in the article.

Filter – the filtration unit as a whole.

Insert – one medium that is ‘slotted’ into’ the filter on its own.

Cartridge – an insert that is made up of more than one type of media.

Media – the material used to make up an insert or a cartridge, such as carbon being used to create chemical media

Do Bettas Need A Filter?

In the majority of cases, yes. Alongside monthly water changes and cartridge changes, a good filter will provide the stable and clean environment your fish needs to thrive. It can also help to oxygenate the water.

Harmful chemicals can be found in aquarium water; they occur for various reasons. A filter will help to break down these harmful chemicals, maintaining a water quality that’s suitable for your betta. A good filter, if properly maintained, will efficiently remove ammonia and other unwanted chemicals from the aquarium. The filter will also mechanically remove any debris from the water — such as excrement — , via a pump system and a straining medium (also known as mechanical media).

This will prevent the debris from circulating in the water. We talk in more detail about how the filter works later in the article.

Can I Avoid Using a Filter?

We understand that some people are resolute when it comes to not using a filter. However, we don’t recommend this and we urge you to use one, especially if you are an inexperienced betta keeper.

Sometimes people don’t want to use a filter because they can be bulky and sometimes don’t suit the look of a betta bowl. However, a bowl isn’t ideal for a betta, and we would recommend using a rectangular tank with a filter. You can read more about this in our ‘can I keep a betta in a bowl’ article.

If you are reluctant to keeping a betta without a filter, consider the points below.

What You Should Consider:

  • Will you be willing to do weekly, if not daily, water changes? For an aquarium, without a filter you’re looking at least a 60% water change every 3  days. Even then, weekly water changes can be less effective than filters.

Can I do weekly water changes instead of using a filter?

Some people feel it’s more practical to do water changes rather than use a filter. Not only does this take more time, but it can be less effective. It can stress the fish, and water changes will need to be carried out very often (more than once a month, anyway). A good filtration set up will leave your fish less susceptible to illness and disease. A quality filter plus regular water changes is the optimum setup in the vast majority of cases.

Is waste broken down naturally without a filter?

Biological breakdown of waste does occur in a freshwater aquarium. Fish food, fish waste and other organic matter will break down into a highly toxic compound: ammonia. If untreated, ammonia is lethal to fish, even in very small concentrations. Fortunately, there is naturally-occurring bacteria in the aquarium that will break down ammonia into less harmful compounds. These are called nitrifying bacteria.

The bacteria (Nitrosomonas) breaks ammonia down into nitrite. A different type of bacteria (Nitrobacter) breaks the nitrite down into a compound called nitrate, which is far less harmful to fish. Nitrate content can be kept at safe levels by performing partial water changes (20% every 1-3 weeks). The bacteria mentioned above will thrive on the foam and biological media included within a filter anyway.

This is why water changes can be less effective than filters. Chances are you may remove vital bacteria needed for the benefit of the fish, or you may not be removing enough chemicals like nitrate, which could build up and and cause harm to your betta via illness and stress.

Is a Filter Bad For a Betta?

No. If you use the right filter in the right environment it’s the complete opposite — it’s healthy for your betta. It’s true that bettas don’t like water movement and yes, a filter does produce a current with its pump system, but there are filters on the market with weak / adjustable outputs. In fact, there are aquariums available especially made for bettas that come with a compact maintenance system, including a filter. You can see some of these aquariums in our ‘best betta tanks’ article.

How Does a Freshwater Filter Work?

So you have a filter attached to the side of the aquarium, or you’re thinking of buying one for your betta, and you’re probably wondering, “How does this little machine keep things clean?” It may seem complicated at first, but it’s fairly simple once you know what’s taking place.

With most modern, run-of-the-mill aquarium filters, water is sucked into the filter via a pump. The input pump is usually located at the bottom of the filter underneath the output pump, or with some filters the water enters at the surface of the water via an overflow. The water in is then pumped upwards, downwards or sideways through the filter’s filtration media, which can take the form of either separate inserts, or a cartridge (see our definition and diagram above for more info).

A filter cartridge can be comprised of 2 or 3 different mediums. The three different types of media are biological, chemical and mechanical. Let’s take a look at the job each one does.

Functions of Mechanical, Biological and Chemical Mediums

Mechanical – mechanical components, such as a simple sponge, physically strain waste solids (fish waste or leftover food) from the water. They essentially ‘catch’ the waste products as they pass through, which means that the other filter mediums can work more effectively. The mechanical component of a filter does not change the chemistry of the aquarium water, although it does provide a home for beneficial bacteria in the filter as well.

Biological – biological media is a substrate with a large surface area (usually ceramic) that’s inserted into a filter in order to create the perfect home for beneficial bacteria. This bacteria breaks down solids and wastes to create a less toxic product. Bio-media doesn’t need to be replaced as often as chemical media.

Chemical – specific chemical inserts can be used for a range of filtering purposes. They remove different impurities from the water. Sometimes it is preferable to have a chemical medium in a filter for a betta, other times it isn’t (such as when you want to promote encourage plant growth). We cover this in more detail later in the article. Typically, standard fresh water filter systems come with carbon media.

How the water passes through the filter

Now that we’ve covered what each filtration medium does, let’s take a look at how the water actually passes through the filter and the different mediums.

Water usually enters a filter through a pump or overflow. The water then passes through the various filtration mediums. The water may be pumped up, down or sometimes sideways through the mediums, as long as it passes through all of them.

Water is usually drawn from the main body of water through the  bottom of the filter, but can sometimes enter from the surface of the water via an overflow. The water usually exits the filter at the surface of the water — this is  to help with water oxygenation.

In terms of passing through the filtration media:

First, the water will be passed through the mechanical layer. This is a sponge-like foam that will simply catch any solid matter (like fish waste, uneaten food) and prevent it from re-entering the the aquarium water.

The next layer is usually the biological layer. Biological media is a substrate with a large surface area on which beneficial bacteria (which helps to break down waste products in the aquarium) can be cultivated. The water passes through the media and somewhat ‘force feeds’ the bacteria the waste products from the water. The beneficial bacteria that grows on media is important, it can sometimes take a year or more to cultivate an efficient batch of bacteria. Of course, it’s not JUST the biological media that the bacteria grows on in the aquarium, it grows on all surfaces good and bad, but the media in the filter has a concentrated, efficient batch.

The third layer is the chemical layer. This is usually a sponge impregnated with a removal substrate (carbon being a common one with freshwater filters), or it can be a sack containing charcoal. The carbon helps to remove unwanted or harmful chemical from the water.

To note: these mediums can be layered in different orders, or sometimes there will be two of the same media at different stages of the filtration. For example, sometimes mechanical media can be a placed at the beginning and the end of the filtration process.

Once the water has passed through all the layers of the filter, it is then pumped back into the aquarium.

To note: the removal substrate from any chemical layers needs to be replaced every 3 to 6 weeks as the waste matter inside the filter will build up. Many manufactures will make filter matter easy to replace. This is  sometimes done via a removable cartridge so it’s as simple as old cartridge goes out, new cartridge goes in. We cover this in more detail in our best filter for a betta article.


To help you understand how filtration works, we have created the diagrams below that explain each step of the process.

The first diagram shows an ‘add-on’ filter, the Tetra Whisper 10i, that contains a cartridge. This is a filter that would be purchased separately to the aquarium.

The second diagram shows a built-in aquarium filter (which in this case is part of the the Fluval Spec 19L), which has separate filter mediums, not a cartridge.

Filtration with a separate filter (Tetra Whisper 10i)  that contains a cartridge:


  1. Water enters the Tetra filter through the lower intake – this sits inside the aquarium, submerged. The water is pumped up and into the filtration compartment, which is outside of the water.
  2. Within the filtration compartment, the water enters the 1st chamber. The water is pushed down through the cartridge, which is made up of mechanical and chemical media – in this case floss and carbon. The mechanical medium holds solids (waste and debris) and the chemical medium removes carbon dioxide and unwanted chemicals from the water.
  3. The water now enters the 2nd chamber. The water is pushed up through the bio media. The bio media houses beneficial bacteria, which helps to break down waste chemicals. In this case the bio media is not ceramic, it is one piece of material with a large surface for bacteria to grow on.
  4. The water exits from above through the overflow, which sits in the aquarium just above the surface. The water trickles back into the aquarium. When the water runs back into the aquarium and hits the surface of the water, this causes air to mix with water. Oxygen from the air diffuses into the water, which helps to oxygenate the aquarium.

Built-in Filtration: (Fluval 19L)


  1. Water is pulled into the filter through the overflow grid towards a pump located at the bottom of the filter.
  2. The water passes through an initial sponge/mechanical layer. The whole of the filtration section is structured within a sponge insert, which you can see in the image above. This insert counts as the mechanical media.
  3. Next the water gets pulled through a chemical media insert, which can easily be replaced and removed.
  4. Then through another layer of sponge.
  5. Then through a bio media insert.
  6. Then through a final sponge/mechanical media layer before it’s pulled into the pump where it is pumped up and out. The filtration pump can be adjusted to give a strong or weak output. We recommend turning the water output to the lowest setting because bettas don’t like a strong current. It also means the water won’t break, which tends to make bettas uncomfortable too. If you are using a Fluval Spec for your betta, set the output to it’s lowest flow setting by pushing the switch all the way over to the right.
  7. The water exits on the right at the surface through a return nozzle. The nozzle output direction can be adjusted, enabling you to control where the current is focused.

Filter Cartridges and Maintenance

Maintenance is an important aspect of filtration. How easily you can clean and maintain your filter and its media will be a consideration when you are choosing one to buy. In our best filter for a betta article, we take this into consideration with the filters that we recommend.

When it comes to looking after your filter, here are the factors you need to be aware of:

All filters contain sponge media. This doesn’t need to be replaced as such but rinsed and cleaned thoroughly with fresh water, maybe every 4 — 6 weeks. Typically it’s a good idea to rinse it every time you do an aquarium clean. It doesn’t degrade — usually standard filter sponges won’t — but it will build up with waste and may become dilapidated after 3 – 4 years. If the waste is never cleaned from the sponge, it will cause your filter to clog and could mean solids will backwash into the aquarium.

The biological medium plays an important role and shouldn’t be changed or replaced too often, if at all. Certain bio medias will disintegrate over time –like ceramic media — so you will need to eventually replace it, but not for a while after fitting (a good 3 to 4 years). We actually have two slots for biomedia in our filtration on the Fluval 19L. One of the slots was originally for chemical media – a carbon sack, but seeing as our aquarium is heavily planted, we removed the carbon and added in more biomedia. We added the second slot of biomedia 6 months after the first, which means that we can replace the two biomedia inserts at different intervals, adding a new insert every year or so . We like to use ceramic media, which does tend to disintegrate but proves to do a good job for our aquarium.

How often you need to change the media in your filter will depend on several factors. Typically the mechanical layer will need to be rinsed monthly, the chemical medium will need to be replaced monthly, and the biological layer will need to be replaced much less frequently. The ability to remove and change filter cartridges and inserts is an important consideration when buying a tank or a filter for your betta. We discuss it in more detail in our ‘Best Filter for a betta’ article.

Chemical Media for non-planted aquariums

Carbon Dioxide (CO2) can build up in the aquarium and become a harmful byproduct. If your aquarium isn’t planted, then we recommend having a chemical medium in your filter. The chemical filter will remove the CO2,  — which is  is created by bacteria, and your betta during respiration — from the water .

The chemical layer of the filter (which is typically made of carbon) removes any CO2 or chemical toxins from circulation. The chemical media doesn’t ‘break down’ the CO2 as such, but absorbs it from the water, and holds it away from circulation.

It can only absorb a certain amount of CO2 (depending upon how much chemical media is in the filter,) and it should typically be replaced every 3 – 4 weeks. If not it will become less and less efficient until it’s rendered useless. The more animals you have in the aquarium, the greater the amount of CO2 that will be released into the water, and the more frequently you will need to replace your chemical medium.

Do you have any questions about betta filtration?

If there’s anything you would like to ask about betta filtration, please let us know in the comment. For more information, you may like to read our other filtration articles: