Buying Guide: Aquarium Filters

If you're new to the world of fishkeeping, you may wonder why a filter is necessary for your aquarium. 

Simply put, an aquarium filter is essential for maintaining a healthy environment for your fish, as it helps remove waste, debris, and harmful chemicals from the water.

In this guide, we'll cover everything you need to know about choosing the right filter for your aquarium. We'll also recommend some of the best aquarium filters available at Complete Koi. Let's dive in!

Read on to discover more about buying the right aquarium filter…

Aquarium filters overview 

When setting up an aquarium, the most important piece of equipment, undoubtedly, is the filter. So, getting the right one is vital for a healthy tank. 

Many aquariums will come with a filter as part of the kit; however, not all are suitable for the intended use. Some filters may need replacing or upgrading, and the following information will hopefully help you choose wisely. 

What is an aquarium filter?

An aquarium filter is a system used to collect and trap waste in a tank. Through an inlet, it will take the water from the tank, pass through the filter media, and then break down the waste into nitrates. It will return the filtered water back to the tank through the outlet.

Most filters will also help to aerate the tank as the returning water from the outlet will promote oxygenation if there is surface movement.

Factors to consider when purchasing an aquarium filter

The two most important factors to consider when purchasing an aquarium filter would be the overall size of your tank and the type of filter you choose. See below for more information. 

The size of your aquarium

When purchasing an aquarium filter, you should first bear in mind the size of the tank. 

Your filter should always be able to cycle the aquarium water efficiently to prevent waste build-up and clarity issues. The filter should say what size aquarium it is meant for, but it’s a good idea to always over-filter where possible, especially if you have high waste-producing fish or a heavily stocked tank. 

For example, if you have a 50L tank, a Fluval U1  aquarium internal filter would be sufficient. However, we would recommend going for the Fluval U2 instead, as it more than covers the tank's needs and has an adjustable flow rate to meet the aquarium's specific needs in the long term.

Aquarium filter type 

The second thing to consider is how you would like to position your aquarium filter. Do you want it in or outside the tank? Would you be better off opting for an external aquarium filter, or is an internal fish tank filter more suitable? 

Both filter systems have pros and cons, but rest assured they are both capable of providing proper filtration to the tank if set up correctly.  

In smaller aquariums, people typically opt for internal filters. For larger aquariums, you’ll more commonly see an external aquarium filter being used. However, this doesn’t always have to be the case. 

Accessibility to the tank and the filter can also play a role in the selection process of your filter. You need to be able to get to your filter to clean it regularly, so it must be positioned somewhere you can get to fairly easily.

Depending on the type of fish you intend to keep, you may need a filter with multi-stage filtration. For example, marine fish (and even some freshwater species!) thrive more when better quality filtration is provided within the setup.

Please see the following section for a more thorough breakdown of filter types.

The different types of aquarium filters

There are hundreds of different aquarium filters out there, but they can be broken down into set categories. There are two main ways to place a filter in an aquarium: internally or externally. The role of a filter is always the same, but the way it is utilised and located within a tank can be different. Hopefully, this breakdown will clarify things for you to see which filter system would suit you best!

Internal aquarium filters 

An Internal aquarium filter is typically used in aquariums under 200L. They are compact systems that attach to the sides of the aquarium. They will usually contain multi-stage filtration (mechanical, biological & chemical). Many of these filters (e.g. Fluval U Series) come with cartridges to be replaced easily and efficiently. 

Moreover, some are customisable (such as the Betta Internal Filter) to be adapted to better suit the tank's unique needs. They are easy-to-manage systems that work really well for beginner tanks.

A great example of an internal filter would be a sponge filter, which tends to be used in conjunction with air pumps to provide a very gentle filtration system for a tank. They are most often used in fry, shrimp, or hospital tanks.

Sponge filters provide fully mechanical filtration, but the inlet flow rate is not as strong as other readily available options. So it’s an ideal internal filter for aquariums where fish need to build strength or may be at risk of being pulled by the flow of the current. The media is also not encased, so nothing can become trapped inside.

External aquarium filters

External filters are often chosen for larger tanks or tanks with a bigger waste load. They are also popular because of the clean look they give the aquarium- no equipment can usually be seen within the tank, and it leaves more space for the fish. 

There are three main types of external aquarium filters: hang-on, canister, and sump. We’ll go into more detail on each option below. 

Hang-on external aquarium filters 

Hang-on external filters are very similar to their internal counterparts, especially in how they are comprised. Their casing is very similar, but they hang outside the tank, and usually, only the intake strainer is inside the aquarium. These are usually popular in nano tanks where you try to maximise the aquarium space but still want to utilise a good quality filter. These filters can be used in the Fluval edge Aquariums and are great for handling the waste of small fish such as tetras, guppies and rasboras. 

Canister aquarium filters

Canister filters are often used in larger tanks (above +150L) for freshwater or marine systems. They can handle large amounts of waste and are positioned externally. They will either be built into the tank frame or use inlet/outlet pipes to move the water to and from the filter and the tank. In addition to the above, they are easy to clean and customisable as they typically have large baskets to hold the filter media. 

Some canister filters even have built-in UV systems (e.g. Betta UV Canister), which provide an extra layer of sterilisation. Some canister filters even have the capacity to filter great volumes of water efficiently. The Fluval FX range is a great example of this! It can filter tanks of up to +1500L, making it a great filter system for huge aquariums.

Sump aquarium filters

Sump filters are probably the most adaptable of the filter systems, but they can also be the most untidy. 

They are like having another tank under your aquarium filled with multiple chambers of different media used to try and achieve what you want from your setup. Sump filters are often used for marine setups but may also be used for larger freshwater aquariums.

Typically, they’re made of a glass/ acrylic frame and use return pumps and outlets for cycling the water between the sump and the aquarium. The filter media is then up to you. A good starting point would be some sponges, filter floss, bio rings and carbon/Zeocarb. Other media can then be added or adjusted accordingly. 

As sump filters have open tops, additional equipment can be added to the system, such as protein skimmers and bio-react units too tall to fit into other filters. The aquarium heaters can even be hidden in the sump compartment to make the aquarium completely equipment free!

Even though these filters can be quite big, they can be easily hidden in the cupboard beneath the aquarium and can be made as basic or complex as you like. 

The three main types of aquarium filtration

Within each filter, there are different media that are responsible for the level of filtration the tank will achieve. The more stages of filtration you have in a filter, the better. You can combine different media types by improving certain filter systems, but some are already curated. 

The most basic three-stage filter systems will have a sponge, bio-rings and activated carbon. But some will come with more or fewer media depending on the quality of the filter.  

There are three main categories of filtration: mechanical, biological and chemical.


This is the media found in most aquarium filters. It is sometimes the only stage of filtration. The ‘mechanical’ element refers to the sponge, but many different types can be used. 

Generally speaking, a thick, coarse sponge will be used, but other filter foams and filter floss can be layered in or used instead, depending on the type of fish in the aquarium. 

The sponge catches the thick waste particles as it passes through the filter system. If layered with finer media, the particles caught will be even finer. This is especially helpful in aquariums with smaller fish (that produce small-sized waste) or if there are visible specs in the water causing clarity issues. 


The second stage of filtration. In theory, every surface of your tank acts as a biological filter to an extent, but a biological filter media is a surface specifically designed to house beneficial bacteria, which is left mostly undisturbed. The media can come in many forms, such as biological rings, K1 media, Biomax, Alfagrog, Cirax and many more!

In marine systems, live rock is usually used in a compartment of the filter to cultivate a large number of good bacteria (when a sump system is used) as there is more space. 

This filtration stage is key to the speedy conversion of ammonia and nitrite to nitrate. To prevent a toxic build-up that the tank can no longer manage, it's important to provide a suitable amount of biological media to a filter system that will allow for it.


This is the stage of filtration that is often included to combat or prevent an issue and is another media within a filter system. 

The most popular ‘chemical’ media include activated carbon (which helps with water clarity and removes impurities, contaminants and odours). Purigen - this is a synthetic absorbent media that targets impurities and discolouration. It also helps to control and remove nitrogenous organic waste that could otherwise be harmful to the tank. 

A replenishable phosphate remover (such as Clearmax or Rowahphos) is usually added to an advanced filter system to prevent a sudden phosphate spike from occurring. 

How often should you clean an aquarium filter?

Each tank is different, so each will require cleaning at different rates. However, there are general rules of thumb to go by in relation to filter cleans. In an established tank, the filter media can be cleaned monthly during a water change. 

The sponges should be rinsed in the water taken from the tank and used in the filter until they no longer come clean. This media will need changing the most often (typically every 2-4 months). 

If the sponges are particularly dirty and causing blockages, increase the consistency of the filter cleans but be careful not to overdo it, as this can do more harm than good. 

The biological media should be gently rinsed in the aquarium water to remove the bulk of the waste that may have gathered on it, then placed back into the filter. This is the media that will be replaced the least. It can be replaced every 6-12 months, or, in cases where the media breaks down over time, will simply be topped up when it is running low.

The chemical media should not be washed. It should be replaced when it is no longer beneficial to the tank. Each product is different; refer to the packaging and the instructions provided to determine the average lifespan.

If changing or replacing any of the filter media, ensure you only change one at a time to not upset the concentrated levels of bacteria it contains.  

With any filter or filter media, you should never use tap water! This will kill off any beneficial bacteria cultivated within your filter system and likely increase the ammonia and nitrite levels in your aquarium water. 

After thoroughly cleaning the tank and filter, water testing is always advisable to ensure no parameters have been affected. After a water change, a de-chlorinator should always be used to make the tap water safe for the fish.  

Aquarium filters at Complete Koi 

Although there are many aquarium filter products to choose from, at Complete Koi & Aquatics, we have compiled a list of some of our favourite systems to make selecting a filter that bit easier. See below for our top picks. 

Betta Aquarium Internal Filter

The Betta Internal Filter is a great basic filter for smaller tanks. This reliable filter can be used in its original form, or its media can be adapted to increase filtration quality. Some smaller versions come with a spray bar for an additional aeration feature. 

Betta Internal Filter - shop now.

Aquarium Bio Sponge Filter Double

The Aquarium Bio Sponge Filter Double should be used in conjunction with an air pump.  This superb filter provides a quiet running, and easy-to-clean system. With a gentle water flow and aid to increase oxygen levels, this effective sponge filter option will work especially for shrimp, fry, and hospital tanks. 

Aquarium Bio Sponge Filter Double - shop now.

Fluval U Aquarium Underwater Filter

Available in various models to suit your unique aquarium, Fluval U Internal Filters are highly convenient with the cartridges they use. Access is easy to gain through the flip-top opening, making cleaning simple. These internal aquarium filters have multiple outputs for flow regulation that can be adjusted to suit every aquarium's needs.

Fluval U Aquarium Filters - shop now.

Betta Choice Aquarium UV Canister Filter

The Betta Choice Aquarium UV Canister Filter is a multistage external filtration system with a built-in UV steriliser. This canister filter option contains various media in trays and cartridges throughout the canister, providing an excellent level of filtration and clarity for aquariums up to 540L.

Betta Choice Aquarium UV Canister Filter - shop now.

Fluval Aquarium External Filter

Available in various models, the Fluval External Aquarium Filters and the Fluval FX External Aquarium Filters are brilliant for effective multi-stage filtration. Media and cartridges can be changed and altered to suit the needs of an aquarium. These compact systems can be used on aquariums from 130L up to 1500L, making them ideal for all aquariums. 

Fluval Aquarium External Filter - shop now.

If you are unsure or want to discuss further aquarium filter options, don’t hesitate to get in touch. Call us on 01205 582185, and we will endeavour to find the right filter for your needs.  

Aquarium filters - our final advice 

  • You always need a filter - no matter how big or small your tank is, if there are fish, even the most basic filter is better than no filter at all. 
  • Only change one media at a time - bear this in mind when replacing media. You don’t want to remove too much good bacteria from your tank by simultaneously changing too many media elements. 
  • Never rinse media under tap water - when cleaning, always rinse your media in water taken from the tank!
  • Bigger is always better - ensure your filter is big enough for your tank. Over-filtering is always best where possible. 
  • Take some time - to understand your filter and ensure it’s the right one for you; it’s always best to take some time to consider your options. Ask questions about it, read some reviews and ensure it will do everything you need. After all, it’s the most important part of your aquarium set-up, so you’ll want to get it right!

Find the right filter for your aquarium with Complete Koi & Aquatics

We hope you’ve found our buying guide on aquarium filters useful. 

Here at Complete Koi & Aquatics, we only stock products we believe in, brought to you by reputable manufacturers. Our selection of aquarium filters caters for hobbyists of all levels. So, whether you’re a beginner or have spent many years creating thriving aquarium environments, we have the best quality equipment to suit everyone's unique needs and requirements. 

If you want to browse our full range of top-quality aquarium filters, click here.  

Alternatively, this blog is a great guide if you’re looking for advice on buying or changing your aquarium filter. But if you have a question, please don’t hesitate to contact us directly. We’re always happy to help fellow enthusiasts, so click here to get in touch today!

Shop the best aquarium filters at Complete Koi today

For more information and advice on aquatics, check out the rest of our blog… 

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