How aquarium filters work for beginners

Understanding just how your aquarium filter works is one of the major milestones when you begin you aquarium hobby. Essentially, your fish tank filtration system is the key to keeping healthy water, and therefore healthy fish, generating hundreds of calls to the experts at Swell UK each week.

The best information comes free, and we thought we’d walk any newcomers to the aquarium hobby through the basics of fish tank filters.

Staged aquarium filtration

To begin with, we’ll look at the most basic type of filtration found in aquariums – Mechanical Filtration.

Mechanical filtration refers to any part of the filtration process that puts a physical barrier up to remove solid debris from the water. Usually, this can be found in the form of sponges, sieves, foams and matting, all capturing solid matter and particulate such as fish waste, surplus food, debris and anything else that turns your water murky.

Because mechanical filtration doesn’t do much to break down these big bits of aquarium rubbish, the mechanical stage your filter requires the most maintenance from you – clean the foams or matting from time to time with aquarium water, squeezing out the extra dirty water into a bucket. Even more periodically, you will need to replace your mechanical filter media, available from places like Swell UK.

Biological Filtration

The next stage of aquarium filtration is a little more advanced. Most filters feature a chamber containing biological filter media, often in the form of plastic balls with a high surface area to volume ratio. The water simply flows through the network of holes and gaps these balls create, but come into contact with cultures of nitrifying bacteria that colonise the balls (bio media).

This is where the filtration process really comes in:

Your fish create waste in the form of ammonia, the ammonia filled water goes into the biological filter and comes into contact with the nitrifying bacteria, which turns it from ammonia (a dangerous chemical) into Nitrites (still a little dangerous) and then into Nitrates (pretty harmless).

This is probably the most important part of fish keeping. Without adequate colonies of beneficial bacteria in your filter, your aquarium water would have a horrendously high ammonia content, burning your fish’s scales, inhibiting their breathing, feeding algae, and ruining your pH levels.

But while incredibly important, biological filtration takes time to become effective in a new fish tank. This is because the colonies of beneficial bacteria take a while to establish themselves, and to develop into quantities that can be effective.

This can lead to New Tank Syndrome where algae and poor water quality can be a factor. Consider the use of a filter start treatment, and fishless cycling to begin with.


An optional extra on smaller tanks, and considered essential by professional keepers of larger tanks, UV clarifiers are a fantastic extra augmentation for your aquarium filter system. These guys shine a powerful UV light into your aquarium water as it enters the filter, forcing algae particles to clump together so they can be caught by your filter media and broken down, curing most cases of Green Water, leaving you with crystal clear results.

Fluid Bed Reactors

This is where things get complicated, and unless you are trying to create something a bit special, most tanks won’t require a fluidised reactor, but we thought we best cover them a little anyway. Its best to think about aquarium reactors in the same way you think about a biological filter. They work in a similar way, but the difference is that the media is constantly on the move due to a current passing through the reactor, putting extra energy and oxygenation into the process to enhance the processes inside.

Georgina Posted by on

Georgina is a member of the Swell UK marketing team and has been keeping tropical fish for a number of years now. Her favourite fish being the stunning, male Siamese Fighting Fish. She is also looking to expand her existing collection to include keeping saltwater fish as well. Her other pets include Bengal cat, Walter, and Labrador and Rottweiler cross, Presley.


  • Avatar shirley wildgoose Posted 05/01/2016 at 5:57 pm | Permalink | Reply

    I have just set up my tank and am keeping coldwater fish. My tank has gone very cloudy, I have used the starter liquids recommended. I am not sure which is the most beneficial setting for the filter. Should it produce a lot of bubbles, or is this what is making my water cloudy.



    • Georgina Georgina Posted 07/01/2016 at 12:02 pm | Permalink

      Hi Shirley,

      The water is likely to be going cloudy as it’s still cycling (if the filter is under six weeks old).

      It is advisable to feed once every two days, and do a 20% water change every four days. Also, make sure there are not lots of fish in the tank (for a new tank approx. six tetra sized fish per 180 litres to start off with).

      Also be sure to test the water and check that ammonia and nitrate levels are not too high.

      I hope this helps

  • Avatar paula Posted 11/01/2016 at 12:16 pm | Permalink | Reply

    I have just noticed that I can see the 2 coloured wires showing through on my tank pump where the black cable is connected as f its been this posing a electrocution risk as at times I do put my hand in water to ground plants back into gravel with the pump being on :/

    • Georgina Georgina Posted 15/01/2016 at 3:06 pm | Permalink

      Hi Paula,

      Please never put your hand in water if you suspect that a product may have an electrical fault.

      Un plug the item at the mains then remove from the Tank/Pond and get qualified advice.

      I hope this helps.


  • Avatar Davina Burness Posted 29/11/2017 at 12:04 pm | Permalink | Reply

    did u wash the stones separately before u put them in the tank

  • Avatar Louise Posted 30/12/2018 at 1:51 pm | Permalink | Reply

    I have a swimming area for my water dragon it is 2.5 foot wide and 2 foot long and 1.5 high. I am using a internal power filter. The problem is the dragon takes in a lot of soil so water dirty. Any suggestions greatly appreciated.

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