The Basics of Filtration

This user guide has been written to outline the importance of filtration, and to provide a basic understanding of how filtration works. This guide is aimed at new comers to fish keeping and will not include more complex forms of filtration, such as those associated with keeping marine fish and inverts.

Let's start with the obvious, why is it necessary to filter our fish tanks? If we keep livestock such a fish or plants, waste products will naturally be produced. As our fish tanks have a limited volume of water, these will build up over time, polluting the water within which our livestock live. Aquarium filters are used to remove or inhibit the waste products that are produced, helping to maintain a safe and non polluted environment.

So how is this achieved?

There are three fundamental approaches to filtration: mechanical, chemical and biological.

Mechanical Filtration:

Mechanical filtration is the process of removing debris e.g. plant matter, food debris, etc., from our fish tanks. Water is forced through either a foam or a polyester sheet which captures the debris. These can then be removed manually by the fish keeper.

Mechanical filtration is the most basic form available and the bare minimum when keeping any livestock in an aquarium. The most simple form of mechanical filter would be an internal power filter, where water is drawn into the filter through a sponge which traps debris, before being re-circulated within the aquarium. The finer the filter foam, the finer the particles that are captured.

Mechanical filter pads do need to be replaced regularly as over time they become clogged. If using an internal power filter for example, you’ll notice that the water flow from the filter will reduce. This is an indication that the mechanical filter media needs either cleaning or replacing. The service interval of such a filter will depend upon the type of aquarium that you have and how heavily stocked your fish tank is.

Chemical Filtration:

Chemical filtration can be used to remove specific compounds, discolouration and/or odours from your aquarium. The most popular chemical filter media is activated carbon, which can either be used as granules or as foam sheets that are impregnated with carbon and can be fitted within your internal or external power filter.

Over time aquarium water can discolour, turning yellow. Products such as activated carbon remove this discolouration, leaving crystal clear and odour free water.

If you were to look at a piece of activated carbon under a high powered microscope, you would be able to see that the surface is in-fact covered in tiny fissures and cracks. The molecules that are associated with either discolouration or odours become trapped in these tiny fissures, leaving crystal clear water.

You may not know this but Activated Carbon is often used by the water utilities to complete the purification of our tap water, prior to them adding chlorine which acts as a sterilisation agent. This is one of the reasons why the majority of us in the UK have crystal clear and odour free tap water.

Other chemical filters medias are also available, such as Rowaphos or Phosguard, both of which have been designed to remove phosphates from your aquarium water. This can help to reduce the levels of nuisance algae from your aquarium, which are often associated with increased levels of phosphates and silicates. Anyone keeping a marine aquarium will know just how important it is to include these medias with your filtration system!

Chemical filter medias become exhausted over time and therefore need to be replaced at regular intervals, usually every 4 to 6 weeks (but not always). Always review the instructions for your chemical filter media to ensure you follow the correct replacement regime.

One point to note is that chemical filter medias need to be removed from your filter when using medications, say for example a white spot treatment, as the media will remove the medication from the water!

Biological Filtration:

The most fundamentally important form of filtration is biological. In a traditional fish tank for either cold water or tropical fish, this is often achieved using ceramic filter medias such as Hagen Biomax or Eheim Ehfisubstrat Pro. In reef aquaria, live rock is often used.

Fish excrete Ammonia as a by product, and any matter that decays within the aquarium, such as uneaten food or plant matter will also turn into Ammonia – which is toxic to all of our live stock. Luckily, beneficial bacteria exist which will consume the Ammonia, turning this as a by product into Nitrite. This is the first stage associated with biological filtration. Whilst Nitrite is less toxic than Ammonia, it will still harm your fishtank’s inhabitants. A second species of beneficial bacteria however will break down the Nitrite, with the by product being Nitrate – which is again significantly less toxic than Nitrite. As such, there are two types of what we refer to as beneficial bacteria, without which we would struggle to keep any aquatic life healthy in our aquariums!It is at this point I will mention ‘new tank syndrome.’ Unfortunately it can take many weeks, even months to build a colony of beneficial bacteria within your aquarium. Products are available such as Waterlife Biomature which help to develop the beneficial bacteria (referred to as cycling the filter), but again this can take several weeks and often the new comer to the hobby will not be aware of this issue. As such, it is important to stock any new aquarium slowly, to ensure that beneficial bacteria levels have the opportunity to naturally develop, removing Ammonia and Nitrite from the water. If this is rushed, the beneficial bacteria will not be able to deal with the Ammonia and Nitrite produced, a surplus will develop and your livestock are likely to suffer (which is new tank syndrome).

So always remember, don’t add too many fish at any one time – and don’t be upset if your local aquatic outlet suggests you take your time stocking your tank – they’re trying to ensure that the livestock they provide you with prospers, maximising your success rate in the hobby :-)

OK, so I’ve mentioned Ammonia turns into Nitrite and then Nitrite turns into Nitrate. Now for the next phase, Nitrate is still toxic, so how to deal with this! There are a number of options available, such as filter media designed specifically to create anaerobic conditions in which Nitrate consuming beneficial bacteria can grow, or chemical medias designed to capture nitrate – but I’m not going to go into this. Instead , here’s the secret to fish keeping success, regular water changes!

Undertaking regular water changes using dechlorinated water will help to prevent levels of nitrate from increasing over time. It’s as simple as that! And to help you spot a problem developing, make sure you test your aquariums water. There a number of test kits that are economically priced and easy to use. If you spot the nitrate levels creeping up, review the filters you’re using and the amount of food that you’re feeding your livestock, and do more water changes.

Time for one final point which I can’t stress too often. Our tap water contains chlorine that is used to ensure the water that we drink remains sterile as it passes from the water treatment works (you can tell I used to work for Severn Trent Water!) to our tap. This is great for us humans, however terrible news for our beneficial bacteria. So here’s the point I make whenever possible, DO NOT RINSE YOUR FILTER MEDIA UNDER THE TAP! Honestly, you’ll be surprised how many people still do this. Instead, tie your filter maintenance routine to your water change routine, rinsing your filter media in used aquarium water rather than tap water. Not only will this ensure the survival of the beneficial bacteria that you’ve spent months growing, it also means that you only make a mess cleaning the fish tank once every one or two weeks when moving buckets of water and dripping wet filters from the aquarium to the kitchen sink!

We hope that you’ve found this article useful and welcome any feedback that you may have. As mentioned, this is very much a high level over view of the processes associated with basic filtration, not going in to the pro’s and con’s of live rock systems or fluidised filters. If you have any question however please feel free to contact us, and I’m sure we’ll cover the more technically challenging or specialist filtration types in articles in the near future.