The Bio-filter is a living entity and can be considered as the real heart of a recirculating system.
It is needed to reduce the concentration of the toxic part of the Nitrogen compounds in solution in the water.
Nitrogen is an essential nutrient for all living organisms and is found in proteins, nucleic acids, adenosine phosphates, pyridine nucleotides and pigments.
In the aquatic environment, nitrogen is of primary concern as a component of the waste products generated by rearing aquatic animals.
Fish expel various nitrogen waste products through gill diffusion, gill cation exchange, urine and feces.
The decomposition of these nitrogenous compounds is very important because of the toxicity of ammonia, nitrite and, in some cases, of nitrate.
The process that takes place in the bio-filter is called “Nitrogen cycle” or nitrification where safe bacteria (nitrifying bacteria) naturally perform the successive oxidation of ammonia to nitrite and finally to nitrate.
Ammonia is the principal nitrogenous waste as the major end-product of protein catabolism.
A second source of Ammonia comes from the so called “Mineralization” performed by Heterotrophic bacteria that reduce faecal matter and uneaten food into Ammonia.
More than 50% of the waste produced by fish/frogs is excreted as un-ionized ammonia.
Ammonia is a highly toxic compound even at low concentrations.
In water, ammonia exists in two forms: un-ionized (NH3) and ionized (NH4+) and is usually measured as TAN (Total Ammonia Nitrogen) in mg/l. or ppm.
TAN = NH3 + NH4+
The un-ionized form (UIA) is the toxic fraction.
The relative concentration of each of these forms of ammonia is mainly a function of pH and temperature.
An increase of pH and temperature increases the proportion of the un-ionized form of ammonia.
Ammonia toxicity is particularly problematic in newly established recirculation systems, overcrowded aquariums as well as in shipping containers.
Fish suffering from ammonia toxicity may exhibit behavioral abnormalities, such as hyper excitability, increased respiratory activity, oxygen uptake (gasping at the surface) and increased heart rate.
Even relatively short exposures can lead to skin, eye and gill damage.
High levels can also lead to ammonia poisoning by suppressing the physiological exchange through the gills with damage to the internal organs.
The fish response to high ammonia levels can include lethargy, loss of appetite, laying on the tank bottom with clamped fins, or gasping at the surface.
As a result, fish may experience a reduction in hatching success, reduction in growth rate, abnormal development and increased susceptibility to pathogens.
When levels of Ammonia rise, the Ammonia oxidizing bacteria (AOB), belonging to the genus Nitrosomonas, oxidize Ammonia into nitrite.
Nitrite is the intermediate product of the Nitrification process from Ammonia to nitrate.
Although it is usually converted to nitrate as quickly as it is produced, a lack of its biological oxidation can end in a high level of nitrite, potentially toxic to the animals.
Nitrite toxicity is the cause of the so-called “brown blood disease”, having a direct impact on the ability of blood hemoglobin to carry the oxygen.
When it enters the blood stream, nitrite oxides the iron in the hemoglobin from the ferrous to the ferric state.
The resulting molecule is called methemoglobin, which has now a characteristic brown color.
In conclusion, this results in hemoglobin incapable of combining with oxygen, thus hypoxia occurs.
Other N-bacteria belonging to the genus Nitrobacter (NOB – Nitrite oxidizing bacteria) oxidize nitrite into nitrate. Nitrate is the end product of nitrification and is the least toxic of the nitrogen compounds.
Excess of nitrate causes stress making a fish’s organs work harder to adjust to their environment.
The increasing stress results in the fish losing the ability to fight diseases, heal themselves and reproduce.
Ideally no more than 50ppm should be present but the lower the better.
But since nitrate is the result of the nitrification process, it will always be present.
The only way to reduce it is by periodically changing the water: that is why recirculation systems are commonly known as: “semi-closed recirculation systems”.
The N-compounds can be checked either by using “colorimetric testers” or with a photometer.
They should be checked daily (at least for the first 8 weeks of biofilter activation) and at least weekly once it is stable.
The advised levels for N-compounds are:
• Ammonia: 0.01 - 0.1 ppm
• Nitrites: < 0.2 ppm
• Nitrates: < 50 ppm
These values refer to a stable and mature biofilter.
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