Why is filtration necessary?
Why filter the water? The answer is quite obvious when one considers what happens in an aquarium. A fish is a living organism; it eats, excretes in, and “breathes” from the water that surrounds it.
The biology of the Aquarium
Fishes waste products may be solid (eg faeces) or dissolved (eg urine, excess salt excreted from the gills). Solid waste is predominantly made up of organic carbon compounds. Heterotrophic bacteria feed on these, consuming oxygen and producing carbon dioxide. Without adequate oxygenation this process can cause problems, however, most aquarists wish to remove solid wastes simply because they are unsightly. Solid wastes are removed primarily by mechanical filtration. While this clears the water it should be remembered that the solid wastes are not truly removed (and will still be consuming oxygen) until the mechanical filter media is cleaned or replaced.
Dissolved waste products contain nitrogen in the form of ammonia and urea, which is also broken down into ammonia. In water, ammonia exists in two forms: Compound ammonia, NH3, and the ammonium ion, NH4. It is common for these to be referred to collectively as “ammonia”; however, it is the NH3 form that is of greatest concern to aquarists. Ammonia in the ionic state (ie NH4) is relatively non-toxic, while the compound ammonia (ie NH3) is highly toxic to fish. The proportion of dissolved ammonia that exists as NH3 depends on the pH of the water (a higher pH means more NH3) and, to a lesser extent, the temperature. Therefore, at higher pH, ammonia is more likely to cause problems.
In an aquarium, as in nature, ammonia is oxidised by aerobic nitrifying bacteria such as Nitrosomas spp and Nitrospira spp. Nitrosomas spp. oxidise ammonia to nitrite (NO2). Nitrite is still toxic to fish, although less so than ammonia. Nitrite is then further oxidised to nitrate (NO3) by Nitrospira spp. (Note, oxidation of nitrite in aquaria was formally thought to be carried out by Nitrobacter spp, but recent evidence suggests that Nitrospira spp play a larger role than Nitrobacter in aquaria). Nitrate is less toxic than both nitrite and ammonia, although different fish have different sensitivities to nitrate, some being susceptible to nitrate poisoning and some being highly resistant. Corals and many invertebrates are sensitive to nitrate levels.
Nitrosomas and Nitrospira are present in every aquarium. They are present in low numbers in the water, but are mainly found on surfaces: on the glass, on rocks, plants and ornaments, and in the gravel. Wherever there is sufficient oxygen, these bacteria will be carrying out the oxidation of ammonia and nitrite. Therefore, every aquarium has some degree of biological filtration. However, in order to keep more than a few hardy fish in an aquarium a greater number of these bacteria are needed, and they must be supplied with a constant flow of oxygenated water. This is the aim of biological filtration.
Since Nitrosomas and Nitrospira will grow on any suitable surface most filters designed for mechanical or chemical filtration will also provide some biological filtration. Biological filtration will be impaired, however, by the presence of organic wastes, as heterotrophic bacteria will use up the available oxygen before the nitrifying bacteria. Where a heavy load of organic waste exists, conditions can become anoxic (without oxygen), and biological filtration will cease altogether. In the worst case, anaerobic sulphur fixing bacteria will start producing extremely toxic hydrogen sulphide.
For this reason mechanical filtration media should be cleaned or replaced regularly. Clearly, if the filter media is replaced, its biological filtration ability is lost. The best way to achieve efficient biological filtration is, therefore, to have a biological filter media which is not replaced and which does not collect organic waste, or which can be regularly cleaned of organic waste without harming the desirable nitrifying bacteria.