Fish Tank Filters Explained. Read Before Buying!

Fish Tank Filters

Before even thinking about getting aquarium both saltwater or freshwater there is always one of most important thing to consider and research, and that is a fish tank filter you will use in your setup. Filters are designed and serve purpose to remove decaying matter, the fish’s waste products, dangerous chemicals, excess food, and free-floating particulate from the water. As a fish swims around or eats food, it creates waste. In case the waste is not removed, the toxins that the fish or excess food create can build up to harmful concentrations. With sufficient buildup of toxins, the fish can poison themselves. The first stages of this are known as ammonia stress. When it gets fatal, it is known as ammonia poisoning. Also, the organic and decaying matter may make the aquarium cloudy and full of algae if not controlled.

This article answers frequently asked questions in regards to types of filters and the best ones to use for your tank setup. Before selecting the filter type, one should first decide the type of livestock they intend to rear. For instance, live planted freshwater aquariums do not require biological filters, but rather require mechanical and chemical filters. In reverse, unplanted, highly-stocked African Cichlid aquarium requires a combination of filters that includes the three filter types.


Three kinds of filtration are required for a healthy aquarium:


Chemical filtration is made possible by the use of chemical or carbon resins that remove toxins from the water. Activated carbon filters are most popular and they aggressively extract the chemicals from the water to the extent that the carbon gets saturated. It is vital to change the activated carbon in the system often.


It is also known as particulate or physical filtration, and it is made possible by the pushing of water through certain filtration media that functions as a strainer. The strainers hold the floating particles that are too big to pass through the media’s openings. The media can be a dense mass of air bubbles, a sponge, aquarium gravel, special filter pads, or filter floss.


Biological filtration utilizes microorganism such as bacteria. These microorganisms convert the waste matter into less toxic substances. Biological filters will convert toxic ammonia (from the decaying matter, the fishes’ waste as well as dead fish) into nitrite and also toxic nitrite into nitrate.


Now lets divide fish tank filter into two categories:

  • Internal tank filters
  • External tank filters


Internal tank filters

They come in several different styles. They are usually placed in the aquariums; either mounted on the sides with suction cups or put on the substrate. They include the sponge and corner box filter models. For most of the internal filters, it is necessary to attach an airline to the filter inlet; which is driven by an air pump placed on the outside of the aquarium. Air is usually pumped into the bottom of the filter, and it moves through it, then it is released through the filter top where it is let loose into the water surface. The air bubbles from water movement go through the filter which makes filtration possible.

Internal filters are mainly of three kinds:

Corner box filters

Just as the name suggests, it is made to fit the corner of the aquarium. These are usually small aquarium filter that has openings at the top and the bottom. These openings allow water to flow through. The corner box filter is placed on gravel and air pumped into the filter through the bottom and escapes from the top.

Sponge models

This one features a design whereby a u-shaped pipe is used to pull water through a sponge-like material that has a large surface area. An isolated pump is required to offer air flow.

Under gravel filters

Such filters have a slotted plate that is placed below a substrate, such as gravel. It contains several tubes that are known as uplift pipes, which spread upwards towards the water surface. It is necessary to have a provision that aids in drawing the water through the substrate to the tubes. A stone placed in the uplift pipes or a power head put at the top of the compartment can be used.

External tank filters

A typical external filter is composed of a closed canister connected to the aquarium through an inlet tube and an outlet tube. External filters are usually larger and stronger than internal filters and thus can hold more media. They are also easy to clean, and the support is greater than for the internal filters. Considering their superior power, external filters are suitable for large aquaria, large fish, and heavily-stocked aquaria such as the African cichlid tanks.

There are three common kinds of external filters

Power filters

This term is used to refer to a wide range of filters that are made to hang on the back of aquariums. Most of such filters usually employ the three major types of filtration thus can be maintained easily. The units also include a pump that is necessary for drawing water into the filter, and they are usually fully self-contained. The water in the aquarium is pulled using a u-shaped piping that flows via a cartridge and other kinds of filter media. Most of such models require filter cartridges that contain activated carbon. The activated carbon used should be specifically designed for this purpose.

Canister filter

They are pressurized filters that are usually placed below the aquarium. Usually this is best fish tank filter one can buy and should always be used as aquarium filters for large tanks. They execute all the three kinds of filtration. They can either be a complete unit or a modular unit. The entire unit contains an inbuilt pump while the modular unit will require an additional pump for it to work. The modular units come in handy when included with other units such as the wet/dry filter. On the other hand, the complete unit utilizes a u-tube as the water intake and a spray bar for removing the water from the system. Upon installation of the scheme on the aquarium, the water from the aquarium is siphoned to the canister filter. Before entering the filter, the water is first passed through a mechanical media (such as pads or floss) then it goes through a chemical filter. Once the chemical filtration is over, the water goes through the biological filtration, where nitrogen cycle is done before the water returns to the aquarium.

Wet/ dry filters

Just as in the canister filters, these are also located below the aquarium. An overflow device is provided, which controls the amount of water supplied to and from the filter. The overflow device comes in two forms; one is inside, and the other is outside. A u-tube is used to siphon the water from the box that is inside the aquarium to that which is outside. The overflow box situated outside may be lowered or raised; which leads to a change in the water levels inside the aquarium. Once the water flows to the outside box, it then passes through a pre-filter media (mostly a sponge), which is used to trap all the large particles. The water then drops to a filter that is below the aquarium and is agitated by mixing it with air. Next, it passes through a biological filter through a spray bar or a drip plate. Next, it goes to the bottom of the compartment which is known as the sump. In the sump, there are chemical media, protein skimmers, calcium reactor and a denitrator. The sump also contains a water pump that is used to return water back to the aquarium. The water pump can be installed using a submersible pump situated inside the sump or using a bulkhead for in-line operation.


Filter media

As for filtering water, it may be achieved in three ways. It may be: biologically, physically or chemically. Mechanical filters are useful in trapping particles and other debris found inside water. At times, they may clog once they have trapped a lot of the rubble. This situation may cause breakdown of the pump. Thus, it is vital to keep cleaning the surfaces of mechanical filters. Common types of mechanical filters include floss, wood and filter wool.

On the other hand, biological filtration is probably the most important of the three, as it prevents the fish from dying due to ammonia poisoning. When filtering biologically, a porous media with a large surface area is utilized. However, the prime factor here is the biological organisms. Biological filtration offers a habitat for nitrifying bacteria. The more the available media, the larger the surface area, and thus the higher the number of organisms that can survive. When cleaning the biological media, never do so under tap water. This is because the chlorine water may destroy the bacteria required to purify the water. Some of the commonly used biological media include plastic balls and ceramic material. The sponge may also work as a natural media as long as bacteria are colonized in it.

Chemical filtration is usually used to purify aquarium water. The most commonly known chemical media includes charcoal and carbon. Carbon not only absorbs the impurities but also makes the water clear and removes odor from it. Activated carbon is preferred since it has a large surface area and is porous. Resins may also be used as chemical filters and can be used to remove nitrate, phosphate and any other organic pollutants such as ammonia. They may be utilized together with other filter media. Chemical filters can also be used in controlling algae.



Providing a healthy environment for your fish is vital. Filtration not only accomplishes this role, but it also makes the aquarium water crystal clear, thus making your tank to look nicer. However, filtration is not the only activity you should carry out in regards to aquarium maintenance. Your fish require frequent clean up, proper feeding and routine checkup so as to detect any issues such as diseases. Due to the advancements that have been made to aquarium technology, for today it is easier to maintain aquarium systems. Before choosing the type of filter to use, remember first to consider the species that you want to keep as well as the number of fish you intend to have. Having found these things, you can now consider the advantages and drawbacks of each system. Considering the cost is also advisable.

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