How much filtration is required?
Clearly, water must be moved through the filter medium (whether mechanical, chemical, or biological) for it to be effective. More water flow means more filtration, at least to a certain extent, but filtration also depends on the surface area and volume of the filter media. The greater the surface area and volume of media the greater the filtration ability. This is true for mechanical, chemical, and biological filtration. For “good” filtration the entire aquarium volume should be passed through suitable filter media at least three times an hour for cold water and tropical tanks. For marine tanks greater turnover is needed, at least five times an hour is recommended (see our marine FAQs for more information). The flow rate can be less than this where a large volume of media is used, eg in canister filters.
Greater water turnover than this is recommended in smaller tanks (where there is a lesser water volume to dilute toxic waste products), or where the types of fish being kept are large and/or messy.
By far the most common method of moving water is the magnetic impellor-driven pump. These pumps are silent and efficient, and economical to run. An air pump can also be used to move water: as air bubbles rise they draw water up with them, creating circulation. However, the water turnover that can be achieved with an air pump is small compared with the output of most impellor-driven pumps. Other disadvantages of air pumps are noise, even the quietest air pumps are far from being silent, and cost, since most air-driven filters require frequent changes of filtration media.
The role of water changes
The final product of a biological filter is, as mentioned previously, nitrate.
In nature, nitrate is consumed by algae and plants, producing plant tissue, and by anaerobic bacteria, producing nitrogen gas (N2) that escapes to the atmosphere. In an aquarium these bacteria are not generally present, and conditions are rarely suitable for the reduction of nitrate to nitrogen gas. Additionally many aquatic aquarium plants are unable to use nitrogen in the form of nitrate, and so nitrate tends to build up in an aquarium.
Excess nitrate in an aquarium is undesirable because of its potentially toxic effects and the fact that it will promote the growth of algae and possibly damage desirable aquatic plants.
Nitrate can be removed from the water, but methods of removal are not practical for most aquariums. Denitrators remove nitrate by reducing it to nitrogen gas using anaerobic bacteria, but these are generally slow, expensive, and inefficient. There are also chemical filter media that absorb nitrate, but again they are relatively expensive and inefficient. Algal scrubbers can be used; these consist of a shallow, brightly lit aquarium full of algae through which the water is pumped. They are expensive and relatively difficult to set up, and while they are invaluable in, for example, extremely large recirculating systems, they are not practical for most home aquariums.
The only method of removing nitrate which has proven successful in home aquaria is that employed in some marine reef aquaria which utilise organisms that colonise the rockwork and sand (often called “live” rock and sand, respectively). Crevices in the rock, and/or a deep bed of sand provide low-oxygen environments that can harbour de-nitrifying bacteria, allowing the breakdown of nitrate to occur. Water circulation moves water past these organisms and prevent annoxia (complete absence of oxygen). While this method has been used successfully, it may be difficult to establish, and cannot be used in freshwater. This system is usually used in combination with protein skimming, which also helps to slow the production of nitrate.
In most cases the easiest and best way to remove nitrate is with regular partial water changes. Partial water changes will also help to stabilise pH, remove other organic wastes and replace a range of lost trace elements.
For these reasons, however good your filtration, partial water changes are always necessary.