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Filters are important as they remove garbage from the aquarium water. This garbage could be excess food, fish excreta and other unwanted particles including plant debris and even dead fish. If waste such as this accumulates in the tank, the levels of nitrogen will increase to dangerous levels and the fish will become quite sick and could well die.
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How do filters work?
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Filters operate by removing unwanted substances by either mechanical, biological or by chemical means. Most filters combine two or more of these filtering processes.
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- Mechanical filters trap suspended particles of waste in some form of filter medium. The media used could be gravel, floss, foam or diatomaceous earth. Mechanical filters do clog and need to be cleaned regularly. Thankfully the newer filters make this job much easier.
- Biological filters are composed of the nitrifying bacteria in the tank that oxidize waste products. Mostly, they change ammonia from the waste material into harmless nitrates which plants in the aquarium can then use as they grow. It takes a while for these good bacteria to colonise a tank and, once they have established, you should be sure to protect them. The bacteria will grow in the gravel, in the filter media and on anything else that you have in your aquarium.
- Chemical filtration involves the passage of aquarium water through a substance that can change the chemistry of the water. Generally, this is only necessary if you are tackling a particular problem. Activated carbon is commonly used. Less commonly, ammonia-adsorbing clays and ion-exchange resins are also used for specific problems. Carbon will adsorb a range of organic nasties but it will also eliminate any chemical that you might add to the tank to treat fish ailments, so, if you are treating your fish for any disease, you need to turn off or remove any filter that contains carbon.
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Types of filters
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Filters used in aquaria are either internal or external filters. Internal filters sit in the water and external ones are located on the outside of the tank.
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Corner filters
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Filters come in many forms and each has its own advantages. For small aquaria the triangular shaped corner filters are inexpensive. They use a floss material as a mechanical filter and activated carbon to act as a chemical filter. Adding a small amount of gravel to the filter to provide a home for the essential nitrifying bacteria is advisable. As with all filters, a corner filter needs regular cleaning. This involves replacing the floss and carbon but the gravel should not be cleaned, as this will remove the essential bacteria needed in the tank.
Corner filters are cheap but, by their very size, have only a limited filtering ability.
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Submersible filters
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Submersible pump and filter combinations are now very popular. These filters consist of a pump and a filter cartridge in a canister. You can remove the canister separately when cleaning is needed without disturbing the pump and its connection to filter tubes and wires. The cartridge will eventually contain a healthy population of bacteria that will provide the effect of biological filtration. Therefore, when cleaning the cartridge, it only needs rinsing under warm water so that the bacteria are not removed. The charm of these submersible units is that they are compact and easy to maintain.
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Under-gravel filters
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Under-gravel filters are popular and efficient if established properly. These work by having a perforated plate on the bottom of the tank that is then covered with at least 7.5cm of gravel. An air pump or power-head water pump is used to move water down through the gravel and then up through vertical tubes rising from the plate. The gravel soon becomes colonised with a healthy load of nitrifying bacteria that will do a wonderful job of water purification.
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External filters
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External filters are often used too. These attach to the rim of the aquarium. They contain a powerful filter pump that takes water from the tank, passes it through some form of filter medium and then shoots the conditioned water back to the tank. The filter media can be floss or foam inserts or activated carbon. By adding some gravel to this type of filter, nitrifying bacteria will also grow in time to give a biological filtration function.
Your local aquarium shop will help you to decide what type of filter is best for your purpose. Discuss with the aquarist the type of tank you have or are intending to buy and the type of fish you are going to keep. Then take the advice of the aquarist as to what filter unit is best for your application.
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Dr Cam Day BVSc BSc MACVSc is a veterinary surgeon, an animal behaviour consultant and media presenter. In 1995 he qualified as a Member of the Australian College of Veterinary Scientists in the discipline of Animal Behaviour and is one of only 15 veterinarians with this qualification in Australia. He works full time in animal behaviour management in Queensland, consulting with dog, cat and bird owners on a daily basis as well as appearing on air as Brisbane’s radio Pet Vet, and writing for various magazines.
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