Aeration is used in the aquarium to increase the exchange of gasses between the water and the surrounding air. In most cases aeration is primarily used to introduce oxygen to the water, replacing oxygen lost through respiration of living organisms or through depletion from the use of chemical treatments. Aeration is normally achieved through the use of an external air pump, which provides a constant stream of air into the aquarium. The majority of aeration or gas exchange does not occur whilst the bubbles are submerged but when they reach the surface causing agitation and circulation of the surface water. Because most forms of filtration using a flow of water also cause agitation and circulation of the surface water, additional aeration is rarely needed and an air pump is not an essential piece of equipment. There are some specific circumstances in which additional aeration is vital, either through the use of an air pump or additional water pumps situated near the surface. A few of these common circumstances are outlined below:
* Heavily planted aquaria: Although plants are well known to be oxygen producing through the process of photosynthesis, during periods of darkness photosynthesis stops whilst the plants continue to respire, using up oxygen. In situations where this occurs in conjunction with other oxygen reducing factors such as high temperature, high organic load or low surface flow, a heavily planted aquarium can become oxygen deficient at night.
* Use of medication: Some aquarium treatments can 'use up' or reduce waters capacity to hold dissolved oxygen. In other cases medications may reduce a fishes ability to obtain sufficient oxygen, requiring additional aeration to keep available oxygen at a maximum.
* Pollutants or toxins (bad water quality): Many toxins and common pollutants such as ammonia and nitrite can cause an immune response which directly affects and reduces the efficiency of a fishes gill tissue to obtain sufficient oxygen.
* High temperature: The oxygen holding capacity of water is reduced as temperature increases. An increase in temperature also causes a faster metabolic rate