Aquarium filters

How do filters work? 
Filters operate by removing unwanted substances by either mechanical, biological or by chemical means. Most filters combine two or more of these filtering processes.

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. The bacteria that do this job live for a meal of ammonia and thrive on it. Sounds boring to me - give me a Big Mac any day - but for an aquarium, they are very important.

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. Consequently, if you are treating your fish for any disease, you need to turn off or remove any filter that contains carbon.

Types of Filters  
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.

Corner Filters

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.

Submersible Filters

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.

Under-Gravel Filters

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.

External Filters

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.

For information on aquarium management, look for the book Community Fishes by Dick Mills. For the knowledge junkies, the book Aquariology - Master Volume is a detailed reference book. These books are available from your local pet shop.