Basic Types of Aquarium Filtration Systems

Mechanical, biological, and chemical filtration are all important

Goldfish (Carassius auratus) swimming in large rectangular fish tank.
Max Gibbs / Getty Images

A healthy aquarium depends on maintaining clean water, which requires some means of filtration 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 to transform them into 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, or the media in the filter. 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 Biological Filtration?

The biological filter (sometimes called a biofilter) in a fish tank is a filter containing porous media that allows water to pass through it. This filter media serves as a home for aerobic, nitrifying 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 carbon. Other chemical media added to the filter can remove chorine, ammonia or phosphate from the water. The chemical media needs to periodically be replaced.
  • Mechanical filtration is what most people think of as true filtration—machinery that removes solid particles from water by circulating water through filter membranes that strain out solid particles. It is important to understand that mechanical filtration alone is not sufficient since it does not remove or detoxify ammonia 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 particulates 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 filter 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 from which you can choose. 

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 10 gallons or less. Corner filters are often used in separate hospital tanks that are useful to treat sick fish separate from the main aquarium. Their less powerful intake flow also makes box filters popular for use in breeding tanks with tiny fry (baby fish). 

Box filters require an air pump and air line to move water through the filter. These filters create air bubbles that also enhance aeration and biological filtration. 

These filter systems range in price from $8 to $30. 

Canister Filters

Canister filters 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 pressurized to pump water through the filter media, rather than allowing it to flow past it along with the air bubbles as with box filters. This makes them ideal for heavy fish loads. On the negative side, canister filters are difficult to take apart for cleaning and maintenance, and they may be difficult to get primed and restarted afterward. They are the most expensive type of filter, but also work the best.

Canister filters are very good for saltwater aquariums or those with many living plants. Costs can range from $90 to $500. 

Diatomic Filters

Diatomic filter 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. The diatomaceous media must be replaced after each use in a diatomic filter.

Costs of diatomic filters range from $40 to $100. 

Fluidized Bed Filters

Relatively new, these filter systems are very efficient biological filters that utilize sand or silica chips as the filter medium. These units hang from the back of an aquarium, where water is pumped up through a mass of sand or other media. The small particles provide an excellent surface area for the bacterial colonies to thrive. The grinding of the sand as water flows through it helps to break up and trap the particles of debris in the water.

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

Costs for fluidized bed filters can range from $50.00 to $150.00.

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 sponge pad or floss media. Chemical filtration is provided by the water flowing through an activated carbon insert, and biological filtration is offered by beneficial bacteria that grow inside the filter cartridge. Power filters may be combined with a biowheel to provide increased biological filtration. The filter inserts need to be periodically rinsed off or replaced.

Power filters range in price from $10 to $150. 

Sponge Filters

The sponge filter is fitted over a tube from a power head or air pump. The air or water forced through the tube in the sponge creates a suction that pulls water and debris into the sponge material, and bacteria will grow on the sponge to 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. 

Prices for sponge filters can range from $5 to $40. 

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 mesh bags of 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. They are large filters, and are usually housed inside of the aquarium stand.

Trickle filters can be fairly expensive, with costs ranging from $30 to $300. 

UGF (Under Gravel Filter)

The UGF (under gravel filter) is another aquarium filter that has been around for a long time. It utilizes a plastic filter plate that is placed under the substrate, and an air pump attached to tubes on the filter plate that pulls the aquarium water down through the substrate, taking the particulate matter with it. There are water pumps called power heads that can be attached to the uplift tubes inside of the aquarium that are more efficient than operating the UGF using an air pump and air stones.

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. Using a gravel vacuum to keep debris from building up in the gravel bed will help maintain the filter better. The gravel becomes the site of biological filtration as long as water is able to flow through it, under the filter plate, and back up the filter uplift tube. Some UGF have cartridges containing activated carbon that attach to the uplift tubes, which provide chemical filtration, but these need to be periodically replaced ad usually are not necessary to use.

Costs of UGF systems range from $8 to $60.