The nitrogen cycle in the aquarium refers to the biochemical processes in which nitrogen takes part in the aquarium. With the help of the macronutrient nitrogen, plants can not only carry out photosynthesis, they are also able to build proteins. Without nitrogen, there is no growth.

The breakdown within the nitrogen cycle begins with protein-containing organic waste and urea (CH4N2O) being excreted, for example, by the fish and invertebrates in the aquarium. Dead plant material also still contains protein and is likewise part of the waste in the aquarium that has to be broken down.

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The role of bacteria

As probably every aquarist knows, various bacteria in the aquarium are responsible for breaking down waste, mineralizing it and thereby rendering it largely harmless. These and other bacteria involved in the nitrogen cycle like to settle in biofilms on all kinds of surfaces - on the hardscape, meaning driftwood and stones, on other aquarium decoration, on the plants, in the top layer of the substrate, ...

During the mineralization of the protein-containing residues, the CO₂ that was bound during photosynthesis is released again. The nitrogen present in the organic waste is also released again in the course of mineralization, namely in the form of ammonium (NH₄). Urea is also decomposed by bacteria. It is likewise converted into ammonium, provided the pH value is below approx. 7. If the pH value is higher than 7, ammonia (NH₃) is also produced in increasing proportions (the higher the pH value, the more ammonia is present). This is where we enter the so-called nitrification - a very important process in the aquarium.

Ammonium / ammonia - NH₄/NH₃

While ammonium (NH₄) is a largely harmless substance for the animal livestock in the aquarium, ammonia is highly toxic to fish, shrimp, snails and crayfish. A properly functioning nitrification is therefore practically vital for the inhabitants, especially in aquariums with a pH value of 7.5 and higher. In this context, measuring the ammonium level of the aquarium water with a suitable liquid drop test is just as useful as monitoring the pH value in the aquarium.

The pH Alert from Seachem is a practical continuous test for monitoring the pH value.

The first stage of nitrification - ammonium or ammonia becomes nitrite

In the first stage of nitrification, various types of bacteria oxidize ammonium or ammonia to nitrite (NO₂). These so-called nitrite bacteria have names beginning with Nitroso-. They include, for example, the genera Nitrosomonas, Nitrosospira or Nitrosococcus. This group of bacteria is aerobic; they need oxygen for their activity.

Nitrite - NO₂

Nitrite is toxic to fish - it attaches to the red blood cells and thereby prevents the uptake of oxygen. At a corresponding nitrite concentration, the fish in the aquarium can therefore suffocate. A typical warning sign of an elevated nitrite level in the aquarium is fish gasping for oxygen at the water surface. A reliable nitrite test really belongs in every aquarist's cabinet, so that when problems occur you can get to the bottom of the cause right away and do water changes if necessary. By the way, chloride is an antagonist to the uptake of nitrite in fish - with an elevated nitrite concentration, as an immediate remedy you can add table salt to the tank at a concentration of 0.1-0.5 g per liter of aquarium water. In this way, most fish practically no longer take up any NO₂, even at elevated nitrite concentrations.

For shrimp, the nitrite problem looks different. While the robust Neocaridina (that is, Red Fire, Sakura, Blue Dream and co.), which come from pond habitats, and Amano shrimp Caridina multidentata do not react at all even at very high nitrite levels of 1-2 mg/l, even very low nitrite concentrations of 0.3-0.5 mg/l can already lead to an increased susceptibility to disease in shrimp from stream habitats, such as the popular bee shrimp Caridina logemanni. Nitrite levels of 0.5-1 mg/l lead to mass die-offs in these more sensitive species.

The second stage of nitrification - nitrite becomes nitrate

In the second stage of nitrification in the aquarium, nitrite is oxidized further to nitrate. NO₂ becomes NO₃. Here, too, aerobic bacteria that need oxygen are involved. The names of their genera begin with Nitro-: for example, the many well-known Nitrobacter and Nitrospira are involved here, but also the somewhat less often mentioned Nitrolancetus, Nitrococcus and Nitrospina. Here the circle closes: nitrate is taken up again by the plants and everything starts over.

Nitrate NO₃

Nitrate is harmless to fish at the concentrations usual in aquariums; for shrimp, unfortunately, things look somewhat different. NO₃ inhibits iodine in them, which they need to produce their molting hormone. A nitrate level that is too high in the aquarium can therefore lead to molting problems in shrimp. As with NO₂, shrimp from clean, organically unpolluted biotopes, such as the bee shrimp Caridina logemanni and others, are again significantly more sensitive here. For them, an NO₃ level of about 10 mg/l should be aimed for. Depending on the conditions in the aquarium, a higher nitrate level may also work, but with 10 mg/l the aquarist is on the safe side in any case. Less sensitive shrimp from pond biotopes (these include, for example, the species of the genus Neocaridina common in aquariums) and also rather robust shrimp such as the Amano shrimp Caridina multidentata, on the other hand, tolerate a nitrate level of 25 mg/l without any problem. Since the nitrate level often drops toward zero in well-planted aquariums and aquascapes, it is definitely advisable to monitor this water parameter with a suitable liquid drop test and, if necessary, adjust it downward (by water change or other measures as described in the article "Lowering the nitrate level in the aquarium") or upward (with a suitable fertilizer such as Aqua Rebell Makro Basic Nitrat).