Choosing The Right Filter For Your Aquarium

LAST UPDATED 20th of February 2023

This article is written by Pet Circle veterinarian, Dr Emilee Lay BSc (Vet) Hons BVSc and edited by Dr Nicole Wynne Bsc BVMS MANZCVS (Unusual Pets)

Filters are an essential part of fish keeping, but before buying one there are some things to consider. These trusty tools play a huge role in keeping your fish happy and healthy. Every fish tank should have a filter. Before buying a filter its important to understand what filtration is needed for your fish tank, and from there you can decide what type of filter will work best for your aquarium. 

Understanding Filtration 

There are 3 categories of filters:

  • Biological filter: This is the most confusing of all of the types of filters as this actually refers to the bacterial population of the tank that helps to break down fish waste, so not actually a machine or a machine part!
  • Mechanical filter: A filter that removes particles from the water. This is usually the mesh/sponge component within a tank filter.
  • Chemical filter: Uses chemical reactions to help absorb harmful particles 


Biological Filters

These bacterial filters help to break down ammonia formed from waste into nitrite and then into nitrate. They are an essential part of tank cycling (more on that later) and live on surfaced throughout the tank such as glass, ornaments, substrate, and your physical filter. Biological mediums which can be used to house these bacteria include plastic bio-balls or even the sponges used in mechanical filtration. Biological filters can be added to your tank as part of your routine maintenance. 

Mechanical Filtration 

As the name suggests this physically removes waste particles and other contaminants inside your tank. This generally involves sponges or foams which are added to your aquarium filter. Acting as sieves these can trap even the smallest of particles. Remember that mechanical filters only TRAP waste but do not eliminate them. Routine cleaning of your filter is a must. However as you do not want to lose the beneficial bacteria that reside in the mechanical filters, ensure you use tank water to rinse and clean them. Certain Mechanical Filter sponges also have some additional chemical filtering effects as well. 

Chemical Filtration

This is a filter that uses chemical reactions to maintain water quality. A common example of this is activated charcoal, which binds up contaminants in the water, as well as water additives that bind and precipitate specific types of waste. The types of chemicals needed, really depends on your water parameters. Read more about this in Fish Tank Maintenance 101.

Choosing a Filter

The ideal filter combines all three of these and can help generate water flow and oxygen within your tank. It can be overwhelming picking a filter for your tank, especially with the huge variety on the market. The different types of physical filters you can buy include Hang on Filters, Internal Filters, Sponge Filters and Canister Filters. Filters also come in different sizes and strengths depending on the size of your tank. Always check the volume the filter is suited to prior to purchase.

Hang On Filters


Just as they are named, these hang off the back of your tank, and can help oxygenate and provide a strong surface current for your tank. They take up little room but do require filter cartridges specific to their make and model. These are perfect for smaller volume tanks. The tank lid may need to be adjusted to allow them to hang over the edge. It is important to ensure your tank is at an adequate water level at all times when in use. If the water drops to below the level of the inlet of the filter, it won’t work. 

Internal Filters 


These submersible filters are designed to be completely submerged inside the tank. These are perfect for smaller aquariums but do not oxygenate as much as other filter types. These can create a very strong current at the outlet, this can be managed with a spray bar if need be. The main disadvantage is they take up room inside an aquarium. For smaller tanks this may not be ideal. Depending on the brand some require specific cartridges or sponges, but can be tweaked to use most filter media. 

Sponge Filters 


These are super cheap but still efficient filters. Perfect for fry and shrimp, all you need to do is connect it to your air pump. These can also oxygenate your tank water as well. Sponge Filters sit at the bottom of your tank, and can take up much-needed tank space. Although super cheap, there are limitations to sponge filters namely additional media cannot be added and that they need frequent cleans to help remove debris. These can be used in conjunction with other filter types to ensure maximum cleanliness of your water. 

External Canister Filters


These are the juggernauts of filtration. External Canister filters bring the big guns to filtration and water flow. They do require a space outside your aquarium to reside in - normally a cupboard beneath the aquarium - so consider if you have limited furniture space.

They are perfect for larger aquariums. Almost any filter material or cartridge can be added to them. These come with an inlet and outlet that needs to be placed within your tank but do not take up as much space within the tank itself compared to internal filters. External Cannisters may run slightly louder than other filters. Maintenance of External Cannisters can be a bit more finicky compared to other filter types. 

Filter Maintenance 

Your filter and filter media should be cleaned at least once a month for larger capacity filters this might be once a quarter. Remember when cleaning your filter and filter media to use your fish tank water or dechlorinated water to clean it - to preserve the beneficial bacteria in your media. If you need to replace your filter media, ensure you gradually replace it so that the new material can be seeded with beneficial bacteria that already exists in your tank.

Follow your manufacturing instructions as needed to disassemble and reassemble your filter. For filters that have specific cartridges or media inserts, be sure to keep an eye on them as they start to deteriorate so that you can gradually change them over. After cleaning your filter, be sure to add in beneficial bacteria to help support your tank system. Monitor water parameters carefully after a filter clean in case of a marked change to the cycle!

Further Reading

Want to read more? Check out our other articles:

Fish Tank Maintenance and Cleaning 101

Common Aquarium Problems and How to Solve Them

A Complete Guide to Setting Up Your Fish Tank

Cycling Your Fish Tank

Feeding Your Fish

Shop All Aquarium Supplies Now

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