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Why Have Biological Filters in Aquariums?
Fish excrete nitrogenous waste into their surroundings in the form of urea, uric acid, and feces. In the closed environment of an aquarium, this waste compiles without any method of dilution or removal other than filtration and water changes. Since water changes realistically do not happen as often as they should, biological filtration is that much more important to maintaining a safe, stable environment. Even with healthy water changing practices, it is important to detoxify and remove the remaining ammonia for the overall quality of the organisms’ health. In addition to fish waste, organic debris from dead and dying organisms and uneaten feed also contribute to increasing levels of nitrogenous waste. These organic wastes break down and cause the build-up of ammonia (NH₃) and ammonium NH₄+), which are highly toxic to fish. High levels of ammonia can cause osmoregulatory imbalances. This suppresses the fish’s ability to excrete ammonia, leading to kidney, neurological, and cytological failure. It can also cause gill hyperplasia, damaging the epithelia and interfering with the transfer of gases and ultimately leading to suffocation. On top of this, high levels of ammonia will decrease a fish’s tolerance to low dissolved oxygen levels and the affinity of hemoglobin for oxygen, making it even harder for gas exchange to occur. Ammonia toxicity can also cause reduced growth rates and reduced disease resistance All things considered, it is imperative that ammonia levels do not exceed 0.1mg/L, especially for extended periods of time.
When ammonia becomes oxidized, it becomes nitrite (NO₂-). Nitrite, while somewhat less toxic in comparison to ammonia, is still dangerous to fish health. The toxicity is mainly due to the a decrease in the oxygen carrying capacity of hemoglobin in combination with the fact that fish pump this aqueous toxin directly through their gills. This decrease is caused by the oxidizing of the iron in blood (Fe²+ into Fe³+), which prevents it from binding to oxygen. Ideally, nitrite concentrations should remain below 1mg/L in a healthy aquarium.
When nitrite becomes oxidized, it becomes nitrate (NO₃-), which is comparatively non-toxic to fish, only being harmful at levels greater than 100mg/L. It is still important to remove nitrate because high levels of it can negatively affect fish health and development. It is not removed by biological filtration.
In addition to the direct effects of ammonia and nitrite toxicity, the presence of these organic waste products deteriorates water quality and causes stress in fish. Stress leads to poor growth, increased mortality rates, greater incidence of disease, and other negative reactions in fish. Thus, a quality biological filtration system is essential to successful aquarium husbandry.
Biological filtration provides a substrate (called media) for nitrifying bacteria to grow on. Ideally, these bacteria are autotrophic as opposed to heterotrophic because heterotrophic bacteria create ammonia as a byproduct when breaking down organic waste. In a saltwater aquarium, these autotrophic bacteria consist of Nitrosomonas, which convert ammonia to nitrite, and Nitrobacter, which convert nitrite to nitrate.