Articles Water quality

How aquarium filters work

February 23, 2013
aquarium biologische filtratie

Proper filtration is, as most aquarists know, necessary to keep the aquarium water clean and fish healthy. The definition of “proper filtration” varies depending on factors like water volume, the amount of fish, how much water is changed every week and the amount of plants, but in almost every aquarium two different kinds of filtration are needed to avoid problems. Keep reading for more information about what actually goes on in an aquarium filter and why this is so important!

Mechanical filtration

To trap big particles like fish poop, rotting plant bits and leftover food, mechanical filtration with sponge and ideally filter floss is required. Floss is usually placed first in the filter to trap the biggest particles, with the sponge layered from coarse to fine underneath.

Material used for mechanical filtration should be cleaned regularly! Removing debris from the water wouldn’t be very useful if it then just sits in the filter to rot for months, and without regular maintenance the filter flow will also be severely reduced after a while. Gently squeezing the filter material in a bucket of used tank water every few weeks is usually enough to remove the dirt that has been trapped in it over time.

Never clean all the sponge at once though, and don’t clean it in tap water – that could kill beneficial bacteria colonies that have built up in the sponge even though it is primarily meant for mechanical filtration. This also means that when it’s time to replace sponge (when the filter flow is reduced even after cleaning), you shouldn’t replace all of it at once.

Though mechanical filtration can help keep your water values stable and your aquarium healthy, never depend on just sponge and filter floss. The most important part of a filter are actually the beneficial bacteria colonies that live inside it.

Biological filtration

A lot of external filters combine mechanical and biological filtration by layering sponge on top of a bag of biological filter material and topping it off with some filter floss,As mentioned in a previous Aquariadise article on the nitrogen cycle, bacteria are needed to convert the poisonous ammonia (NH3) that fish and other aquarium inhabitants produce into poisonous nitrite (NO2), which is then converted to less poisonous nitrate (NO3) by other bacteria. Most of these bacteria, that are vital to the health of your beloved fish, should live in the filter so they are constantly supplied with fresh, nutrient-rich water and oxygen. A fair amount of beneficial colonies will build up in mechanical filter media like sponge, but these are usually not enough to handle all the ammonia produced.

This is where biological filter material such as bio rings comes in. Designed to allow great amounts of beneficial bacteria to grow and thrive, this stuff is essential when you’re trying to achieve a healthy little ecosystem. In a canister filter, as pictured on the right, both filtration types are combined by layering sponge and filter floss on top of a bag of biological filter media. This produces a super effective filtration system!

There are many kinds of biological media for sale, but what they all have in common is that they should be left alone as much as possible. Disturbing your biological filter media can cause ammonia spikes, so cleaning is not necessary.

Chemical filtration

A third, but less important kind of filtration is chemical filtration, which is usually done with activated charcoal. Activated charcoal used to be considered a vital part of any filter as it removes all invisible pollutants that may be present in the water (a process called adsorption), but nowadays it usually isn’t used ‘full-time’ in filters any more because it also removes important trace nutrients. I personally only use it when I need to remove medication from my aquariums.
Other chemical filtration materials include peat moss (used to lower pH) and zeolite (used to remove excess ammonia).

Sufficient filtration is one of the most important factors when it comes to maintaining a stable aquarium with healthy fish. Although your tank will still need regular cleaning and water changes, doing some research before you buy and getting the right type of filter with the right combination of biological and mechanical media will save you a lot of work and trouble.

If you have any more questions or tips about mechanical and biological filtration, don’t hesitate to leave a comment below. Happy fishkeeping!

Cover photo: The Ensemble by cito17

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  • Reply Vince Leano August 3, 2015 at 10:19 am

    I’m just a newbie in having an aquarium. I have a 15 gal tank with 10 cardinal tetras and 10 neon tetras in it. I use a hang on filter. Besides the activated carbon filter I added a sponge in it. Do you think I can still add some biological filter media in it if space permits? Ammonia spikes concerns me since I do not have a water test kit yet. Does the activated carbon filter lose effect through time?

    Thank you in advance.


    • Reply Mari August 3, 2015 at 10:46 am


      First off, I know this wasn’t your question but your tank is unfortunately a bit on the small side for cardinal tetras. If you have the chance to bring them back to the store and replace them with a smaller and less active fish species (small corydoras are great), it’s a good idea to do so! The neons are fine and a good choice for a 15 gal.
      You can remove the activated carbon filter, you don’t really need it and yes, it loses effect in a pretty short time! You can replace it with biological filter media, but it’s a good idea to get the liquid drop test first so you can keep a very close eye on your water values.

      Good luck! 🙂

      • Reply Gregg Martin October 25, 2015 at 8:26 pm

        I would replace, if you wish, the cardinal tetras with white cloud mountain minnows, a shoal of 6 to 8 in my 15 gallon do well, and they along with the neon tetra do well in cooler temperatures as well.


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