Aquarium Filtration Systems

Mechanical, biological, and chemical filtration are all important

A healthy aquarium depends on maintaining clean water, and doing so requires some means of filtering out the water to remove contaminants and purify the water. Technically, there are three means of filtering aquarium water: 

Biological filtration refers to the process by which beneficial bacteria break down ammonia and nitrite and transform them into compound nitrate, which is much less toxic. For beneficial bacteria to thrive, oxygen-rich water is needed, as well as a surface that bacteria can attach to, such as rocks or sand. All aquariums should have some provisions for biological filtration, and with very small fish populations, this alone might be sufficient to sustain the aquarium. However, in most aquariums, biological filtration will be just one method that is combined with others.

What is a Biological Filter in a Fish Tank?

The biological filter (sometimes called a biofilter) in a fish tank is a cylindrical-shaped filter attached to tubes that allow water to pass through. This filter serves as a home for bacteria that break down fish waste to keep the environment safe and non-toxic.

Chemical filtration is a process by which chemical additives remove dissolved wastes from the water. The most common method for chemical filtration uses activated charcoal.  
Mechanical filtration is what most people think of as true filtration—machinery that removes solid particles from water by circulating water and straining it through some kind of aquarium filter. It is important to understand that mechanical filtration alone is not sufficient since it does not remove or convert ammonia, nitrate, or nitrite in the water. Mechanical filtration serves to remove free-floating waste before it decays into harmful substances, and to be beneficial the filter material must be cleaned or replaced every two to four weeks. In addition to filtering contaminants from the water, mechanical filtration assists in aerating the water.

To effectively maintain an aquarium, a filter should run all the water in the tank through the filter at least four times each hour. When choosing a system, pay attention to what kind of filtration it offers—biological, chemical, or mechanical. Some systems combine the different forms of filtration, to varying degrees of success. 

There are eight common forms of filtration systems you can choose from. 

Box Filters

Also called corner filters or internal filters, these were the first aquarium filters available for home aquariums. Although less common than in the past, they are very inexpensive and can be loaded with a variety of filter media. Many box filters are compact units that stick to the glass inside an aquarium, making them suitable for small aquariums of 20 gallons or less. Corner filters are often used for hospital tanks used to treat sick fish because fish owners don’t wish to invest a lot of money setting up a tank that is used infrequently. Their less powerful intake flow also makes box filters popular for use in breeding tanks with tiny fry. 

Some types require an air pump and air line to produce the movement necessary to move water through the filter. These systems create air bubbles that also enhance chemical and biological filtration. 

Canister Filters

Canisters are powerful mechanical aquarium filters best suited for medium- to large-sized tanks—those larger than 40 gallons. Because canister filters are positioned outside the tank, they may be easily concealed behind or beneath the aquarium stand. These large units provide very good mechanical, chemical, and biological filtration. Canister filters are pressurised to force water through the filter media, rather than allowing it to flow past it as other filters do. This makes them ideal for heavy loads. Adding a biowheel increases the biological filtration capacity of the aquarium filter. On the negative side, canister filters are difficult to take apart for cleaning and maintenance, and they are difficult to get primed and restarted afterward. 

Diatomic Filters

Diatomic systems are specialized aquarium filters that “polish” the water by removing very small particles. In design, these are similar to diatomaceous earth swimming pool filters, which operate by pumping water through a layer of very fine particles to scrub the water clean.

Diatomic filters are most often used in temporary situations when fine particulate matter, such as diatomic algae, is a problem. Because a diatomic filter is used only for special situations, some standard filters are made with diatomic inserts so they may serve a dual function when needed.

Fluidised Bed Filters

Relatively new, these systems are very efficient biological filters that utilise sand or silica chips as the filter medium. These units hang from the back of an aquarium, where water is pumped through it then down through a mass of sand or other media. The small particles provide an excellent surface area for the bacterial colonies to thrive. 

Most units do not come with water pumps, which need to be purchased separately. These units do not provide very good chemical filtration, but mechanical filtration is moderately good because the sand media traps suspended particles. 

Power Filters

Sometimes called hang-on-back filters, power filters are the most commonly used type of aquarium filter, largely because they offer excellent mechanical, chemical, and biological filtration simultaneously. The standard power filter hangs off the back of the aquarium and sucks water up through a siphon tube. They are simple to install and easy to maintain. Mechanical filtration is achieved by water flowing through a filter pad or floss. Chemical filtration is provided by the water flowing through an activated carbon filter, and biological filtration is offered by beneficial bacteria that form inside the filter cartridge. Power filters may be combined with a bio-wheel to provide increased biological filtration. 

Sponge Filters

The sponge filter is fitted over a tube from a power head or air pump. As water is forced through it, bacteria will grow and establish a biological filtration. Sponge filters also provide mechanical filtration, although they clog quickly if there is excess debris. They are excellent for tanks with fry, as the sponge prevents young fry from being sucked through the pump. Sponges are good for a hospital isolation tank, as a sponge from an established aquarium quickly provides the tank with nitrifying bacteria.

When the sponge filters are cleaned, it is important to do so with aquarium water, as tap water will kill the beneficial bacteria that is growing on the sponge. 

Trickle Filters

Also called wet/dry filters, trickle filters are designed to expose the water to as much air as possible. This is accomplished by allowing the aquarium water to trickle over a container of media, such as plastic balls, strands, or floss. This exposure to air and water fosters large colonies of beneficial bacteria that break down wastes. These are especially popular for saltwater tanks, but are becoming increasingly popular in freshwater aquariums, as well. Chemical filtration is provided by placing chemical media in the filter. 

The biggest drawback is the fact that they clog fairly easily. The use of a mechanical pre-filter eliminates or reduces that problem. 

UGF (Under Gravel Filter)

The UGF (under gravel filter) is another aquarium filter that has been around for a long time. It utilises a plate filter that is placed under the substrate, and an air pump that pulls the aquarium water down through the substrate, taking the particulate matter with it. However, biological filtration is limited with this type of system, and chemical filtration is non-existent. 

The UGF is inexpensive, easy to set up, and it is relatively maintenance free once running. On the downside, UGFs tend to clog and are not good choices for aquariums with live plants.

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