The aquarium is a living environment and therefore some natural processes take place. Fish excrete, plant leaves decay and uneaten food rots. All these processes contribute to water contamination and because the aquarium is not affected by the constant cleaning effects of currents, flow and rain present in the wild, the water can quickly become turbid, harbour disease and poison the fish.

Very few aquaria are self sufficient in terms of filtration, that is can mimic without intervention, the complete Nitrogen Cycle. The exceptions are extremely large aquaria with abundant vegetation, few moderately fed fish, and the presence of scavenger fish and algae eaters. However, even those aquaria will still require water changes to replicate part of the Water Cycle. In the days before the sophistication of modern filters, the control of water quality was purely down to water changes and I'm sure we are all aware of the inconvenience that this could cause. So, to prevent the need for daily, or even more frequent, water changes some form of filtration is required.

All filters carry out one or more of the following types of filtration; mechanical, biological and chemical. In addition, a filter can also be used to create currents and provide aeration (oxygenation) through surface movement.

Mechanical filtration removes particulate wastes from the water purifying it as it passes through the filter media. Passing water through some form of sieve, a sponge for example, essentially performs this. The filter media retains dirt and releases water. Nature performs this function as rain filters through strata in the earth. The finer the media, the smaller the particles of waste which will be trapped and consequently the better the mechanical removal of particles from the aquarium. The down side is, the finer the filter material the more prone to clogging it will be. Virtually all mechanical filtering inherently harbours bacteria which grow and perform biological filtration after a period of time.

Bacteria grow on every surface in the aquarium and are fed on the by-products of fish wastes, excess feeding and fish and plant respiration. Biological filtration relies on colonies of two types of bacteria; Nitrosomonas, which break down toxic ammonia or ammonium to nitrite and Nitrobacter, which then convert harmful nitrite to a safer nitrate compound (for further information refer to the Nitrogen Cycle). It may take six weeks or more for enough bacteria to develop and accomplish this task successfully and this period is known as cycling the tank. The filter provides an ideal breeding ground for bacteria as it is there that a constant supply of food (wastes) and oxygen (conveyed by water flow) exists. A vast surface area is needed to house the millions of bacteria necessary to perform efficient nitrification and it follows that the filter media should consist of such a material.

Biological filtration is not so much a means of cleaning the water, as a processes of "purifying" it through the biological conversion of compounds contained within the water. Similarly, chemical filtration alters the water properties via a reaction. Chemical filter media can bond with substances in the water to remove ammonia or ammonium, change the pH or alter hardness. One of the most over looked forms of chemical filtration takes place with every water change when most of us add a water conditioner to remove chlorine or chloramine from tap water. Additionally, the exchange of gases such as the removal of carbon dioxide and addition of oxygen enhanced by surface agitation from a filter outlet can also be defined under chemical filtration but this is covered in greater detail in the aeration section.

By understanding the basic functions of a filter, one is half way to selecting a filtration system to suit their intended aquarium and its occupants. The next step is to choose a filter of sufficient capacity to cope with the size of the aquarium. Most manufacturers helpfully specify the size of aquarium for which a particular filter is designed but as a rule of thumb, one should look at the flow rate of the filter. The flow rate refers to the amount of water that can pass through the filter in a given time. In Europe this is specified in litres per hour (lph). This figure should be 3-4 times the volume of the tank for non or lightly planted aquarium, or 1-2 times the volume for a planted tank. Bear in mind that some fish do not relish a current in the water and subsequently filter with a low rate or one that can accommodate a spray-bar should be chosen. Conversely, for aquariums which contain large or particularly messy fish a larger capacity filter is recommended.

Choosing a filter that is too small for the intended aquarium causes a couple of undesirable effects. Firstly, the filter may not harbour enough bacteria to process all the toxic by-products of the Nitrogen Cycle. Thus the water will become quickly polluted (primarily by with excess ammonia) and poison the fish. Secondly, there is the potential that a small filter may become rapidly clogged. This not only puts strain on the pump, in the case of motorised filters, but also restricts the flow of oxygen necessary for the bacteria to thrive.

As the bacteria dies, it turns anaerobic and inactive in the conversion of toxins. Frequent (but not tougher) cleaning of the filter media can alleviate these problems but why create work? Conversely, one can have too much filtration. This is not harmful in itself but unnecessary. It may also lead the hobbyist into a false sense of security as far as tank maintenance is concerned. Bacteria will multiply and grow on as many surfaces as possible however, there will always be a finite number of bacteria limited by the supply of food (waste products) and oxygen produced in the aquarium. Bacteria like to spread out too. Therefore, if one has a large filter area, the bacteria culture will simply be less densely populated than in an equivalent set up with a smaller filter area. The secret of successful filtration is to maximise and maintain the bacteria culture but the very nature of bacteria colonisation means that whenever one cleans any part of the filter media, regardless of size, some of the bacteria population will be reduced. This is why one should never clean or replace all the filter media at one time. Doing so will mean the cycling process must be started again.

There are however benefits of having a two or more of the same filter, or combination of different filters servicing one tank. When a number of filters are used, and they are cleaned in rotation, there less likelihood of depleting the bacteria culture to such extremes as to cause the effect of a newly set up tank. Similarly if one filter should fail an additional filter will act as a reserve. A third advantage is that a number of filters positioned around the aquarium may service the tank more efficiently in terms of filtering from a wider area in the tank. Also turbulence can be reduced if a number of small capacity filters are used. Finally, a combination of filters can perform different specialised filtration duties. i.e. one may choose to use a diatom filter, which is purely mechanical filtration, to polish the water during maintenance, and a very low maintenance fluidized bed filter to act as the permanent biological filter.

To assist in choosing a filter, I have listed below the most popular types of filter. Each link will take you to a description of the selected filter and explain the benefits and drawbacks of each.