Aquarium filtration
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Aquarium filtration

Mechanical filtration  

Mechanical filters will force the water through some type of filter medium, thereby removing debris, uneaten food, plant particles and similar from the water. The filter medium will normally consist of foam, wool or sand/gravel.

Biological filtration  

Biological filtration is carried out by two different types of bacteria. The first type will convert the ammonia produced by the metabolism of your fish into nitrite. The second type will convert the nitrite into nitrate. Ammonia, nitrite and nitrate are all dangerous to fish, but nitrate is much less dangerous than the other two and well functioning biological filtration is therefore extremely important in the aquarium.

You can increase the amount of biological filtration in your aquarium by providing beneficial bacteria with a lot of suitable surface to colonize. They will for instance love sand, gravel, plants leaves and the filter medium inside your mechanical filter. When you clean the filter medium, never use hot water or detergents since this will kill the bacteria. Ideally cut the filter medium in half and clean only one piece at the time. If something goes wrong, bacteria from the dirty piece will be able to re-colonize the clean one.

Chemical filtration

Filtration may be a misleading word, because chemical filters don’t really filter anything. The most commonly used chemical filter is probably the active carbon filter which will bind a long row of compounds to itself. Active carbon can for instance be placed inside your mechanical filter to remove traces of medication once you have finished treating your aquarium. Other examples of chemical filters are ammonia absorbers and ion exchangers. 

Internal filters

Just as the name suggests, internal filters are filters that are placed inside the aquarium. External filters on the other hand are normally hung outside the aquarium or placed near the aquarium.

Foam cartridge filter
Foam cartridge filters are normally powered by an air pump and will force water through a foam cylinder. In addition to mechanical filtration, this type of filter will carry out biological filtration since the foam serves as an ideal home for beneficial bacteria. Gently clean the foam when it gets clogged or too dirty, but never use hot water or detergents since this will kill the bacteria. 

Box filter
Box filters are very similar to foam cartridge filters; an airstone will force water into a tube and through some type of filter medium, such as foam or filter wool. Clean the filter media just like you would clean a foam cartridge filter. Advanced aquarists sometimes fill their box filters with items that will alter the water chemistry, e.g. peat moss, shell grit, ammonia removers, active carbon and similar. 

Internal power filter
The internal power filter is also similar to the foam filter, but the foam is placed inside a canister and the filter is equipped with a motor to create more forceful water movements. Just like the two filters described above, the internal power filter will be colonized by large colonies of beneficial bacteria if you treat it gently during cleanings. Some internal power filter models have inserts where you can add peat moss, shell grit etcetera.

Undergravel filter
Undergravel filters consists of plates that are placed on the bottom of the aquarium and covered in sand or gravel. The uplifts are equipped with an airstone or similar that in turn can be connected to an air pump. Water will be forced out of the airlifts, causing water to be sucked down through the substrate. The undergravel filter is chiefly a biological filter (the substrate will be colonized by bacteria), but it can also trap debris. For additional mechanical filtration, a layer of sponge, filter wool matting or similar can be laid on top of the plates. Undergravel filters must be cleaned regularly since garbage will collect inside the substrate as well as under the filter. If you do not clean your undergravel filter frequently, it can clog up. 

External filters

As mentioned above, external filters are normally hung outside the aquarium or placed standing near the aquarium.

Power filter
This is one of the most commonly used external filters. It is hung outside the aquarium will receive water from tubes that reaches into the water. The water is filtered through some type of filter medium (foam, filter wool etcetera) and then returned back to the aquarium via a spillway. A fairly strong pump is needed to power this type of filter. Unlike the internal power filter, the external power filter allows you to carry out maintenance work without having to disturb the fish in your aquarium.

Canister filter
A canister filter will normally consist of a closed container inside which water is forced through several layers of different filter materials. In some models, the return tubing will be connected to a spray bar that ensures powerful surface aeration. Canister filters are rather big and are therefore normally hidden away inside aquarium furniture. This way, you only have to see the inlets and outlets. Being up to ten times as big as a normal power filter naturally make the canister filters very powerful, but you should expect to pay several times the price of a power filter.

The bottom layer in a canister filter can for instance consist of bio-balls or ceramic pipes, the mid-layer of active carbon, ammonia removers, peat moss or similar, and the top layer of foam, filter wool, sponge etcetera. Bio-balls and ceramic pipes serves as ideal homes for beneficial bacteria that carries out biological filtration, the mid-layer will perform chemical filtration and the top layer will remove larger and smaller debris by means of mechanical filtration (plus carry out biological filtration once they become colonized by bacteria).

Diatom filter
Diatom filters are equipped with diatomaceous earth that forms a screen on a filter bag. When water is sucked through the bag, big and small particles alike will get trapped on the outside of the bag. Cleaning up using a diatom filter is a fairly quick process since the flow rate is really high. Diatom filters are normally used during maintenance jobs and major cleanings rather than being constantly connected to the aquarium.

Trickle/sump filter
There exists many different variants of this filter, but generally speaking the basic idea will be sucking water from the aquarium and letting it run through a long row of different filter materials. Just as in a canister filter, bio-balls are normally used to provide a home for beneficial bacteria, while foam or filter wool is inserted to provide a combination of mechanical and biological filtration.

Most models are equipped with a sump at the lowest level of the filter where waste will be caught and subsequently removed by the aquarist. Another popular addition is the rotating arm that will spray water over the top of the filter. In earlier models, these arms were inclined to jam and cause problems, and many aquarists therefore prefer a system with fixed spray bars. An algae scrubber or even a small algae pond (popular in saltwater setups) can be included to help keep the levels of phosphate and organic matter down. Advanced models will even have a light above the algae to promote vigorous algae growth. 


When you browse the filter section of your local fish shop you will probably encounter the popular powerheads. Powerheads are not filters in themselves; they are potent water pumps that can be used to increase the capacity of your filter system. You can for instance use a powerhead to power you undergravel filter, thereby augmenting the force of the filter system, increase water flow and reduce the risk of clogging. Powerheads are also used to provide aquariums with strong water currents

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