How does nitrification (nitrogen cycle) in an aquarium proceed?

Nitrification is a process that anyone who is interested in aquaristics is sure to hear about. Is it a difficult to understand? It would seem so, especially if we wanted an in-depth understanding of the nitrification in an aquarium. I assure you, however, that for proper care of the tank you do not need to possess knowledge on an academic level. However, it is important to realize that nitrification is what makes life possible for fish and plants in home aquarium.

The nitrogen in the aquarium

Nitrogen is an essential component of protein, so it occurs in significant amounts in all living organisms. Nitrogen sources in aquarium water are feces of fish, crustaceans, snails, uneaten food remains, fragments of decaying plants and living organisms (mainly microscopic ones as we remove the larger ones). As a result of bacterial activity, organic matter accumulating in the tank is decomposed. Proteins released during decomposition and urea excreted by fish are further broken down.

How are ammonium ions and ammonia formed in the aquarium?

The decomposition of protein and urea results in ammonia, which, depending on the reaction of the water, occurs as ammonium ion (NH4+) or ammonia (NH3).


The contribution of ammonia (NH3) to total ammonia nitrogen increases with increasing water pH. In water with a pH value between 7.0 and 8.3 it is approximately 10%, in water with a pH value of 9.25 it makes approximately 50% of the total ammoniacal nitrogen content. Ammonia is toxic to fish. The lethal concentration for fry is about 0.2 mg/l, and for adult fish – about 2.0 mg/l. As I wrote above, ammonium ions convert to ammonia as the pH of the water increases. In alkaline water aquariums, it is therefore particularly important that the ammonium nitrogen concentration is close to zero, which is directly connected to the nitrification process.

Nitrifying bacteria in an aquarium

Nitrifying bacteria are aerobic organisms and therefore the nitrification process takes place under aerobic conditions. Moreover, they belong to obligatory chemoautotrophs, so the source of energy for them is chemical energy obtained from oxidation of inorganic compounds. In this case, it is the oxidation of NH4+ (ammonium nitrogen) and NO2 (nitrite), while the carbon source is CO2 (carbon dioxide).

Many different types of bacteria are known to be involved in the nitrification process.  Scientific studies indicate that in aquariums the first stage of nitrification is most likely carried out by bacteria of the genus Nitrosospira and the second one by Nitrospira. However, you will find in older literature that Nitrosomonas bacteria are responsible for the first stage and Nitrobacter for the second stage. I’ll stick with this nomenclature, because from an aquarist’s point of view, it doesn’t really matter what specific types of bacteria conduct this process. It is also worth noting that under laboratory conditions nitrifying bacteria grow poorly or not at all, which makes the already difficult process of identifying individual strains even more difficult.

Occurrence of nitrifying bacteria in the aquarium

Nitrifying bacteria are found both in the body of water and on objects submerged in water, on aquatic plants and in the surface layer of the substrate (where oxygen reaches). In many studies of aquatic ecosystems, the highest concentrations of nitrifying bacteria were found on the substrate. In an aquarium we are most interested in their development in a filter acting as a biological filter. That’s why we place there filter beds dedicated to biological filtration. They are characterized by a large surface on which bacteria can settle. Nitrifying bacteria do not form spores and can therefore only survive for a short period of time under adverse conditions.

How does nitrification in an aquarium proceed?

Nitrification is a two-step process. First, Nitrosomonas bacteria use ammonium nitrogen (NH4+/NH3) as an energy source, oxidizing it to nitrite (NO2), and then Nitrobacter oxidizes nitrite to nitrate (NO3). A lot of energy is required for nitrification, which means low nitrification efficiency and slow growth of nitrifying bacteria.

What affects the nitrification process in an aquarium?

Nitrification in an aquarium depends on many factors. As I wrote earlier, nitrifying bacteria are aerobic bacteria. The rate of nitrification may decrease when the oxygen concentration falls below 2 mg/l for Nitrosomonas and 4mg/l for Nitrobacter. Furthermore, too much load of organic matter in the tank (and thus in the filter) may cause competition for oxygen between nitrifying and heterotrophic bacteria, which the latter win. The most optimal conditions for nitrification is temperature 28 – 36°C and the pH between 7.5 – 8.5.

dr inż. (Ph.D.Eng) Aleksandra Kwaśniak-Płacheta

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