Under the nitrite peak is understood in the hunting and in the fish farming a temporal maximum of the concentration of nitrite in water. It often occurs in aquariums shortly after they have been set up or when the stocking density has increased significantly. In fish ponds, it is usually the result of the decomposition of collapsed algae populations. It represents a dynamic transition stage of nitrification .
A newly established aquarium is far from a biological equilibrium: The concentration of substances dissolved in the water changes significantly within days and bacterial populations begin to establish themselves on the surfaces as biofilms . As a rule, the concentration of ammonium ions or ammonia in the water rises first , which is excreted by the fish via the gills as the end product of the nitrogen metabolism of the fish . It is also a decomposition product of proteins from leftover feed or dead algae. This ammonium serves as a nutrient for bacteria of the genus Nitrosomonas , which, due to the abundance of ammonium, reproduce relatively quickly and strongly and oxidize ammonium to nitrite , which accumulates in the water. The now abundant nitrite serves bacteria of the genus Nitrobacter as a nutrient, which further oxidizes it to nitrate . However, these bacteria multiply relatively slowly, so that the nitrite concentration initially increases further. The nitrite concentration can temporarily rise to high values that are toxic for aquatic organisms before it is oxidized to nitrate by the regrowth of the Nitrobacter population. Compared to nitrite, nitrate only has a harmful effect on water dwellers at very high concentrations of several g / l. As soon as both bacterial populations are established, the newly formed ammonium is processed into nitrate practically immediately, without a significant concentration of nitrite being observed.
This concentration maximum (English "peak" for "mountain peak") of the nitrite occurs in aquariums mostly between the second and sixth week after the start of the run and lasts for about a week. Depending on the stocking density, type and amount of feed, pH value of the water, amount of plants, inoculation with bacterial populations, etc. both the time periods and the achieved nitrite values can be very different. After a few days to weeks, the bacterial populations have adapted to the (changed) conditions and ensure a rapid conversion of the ammonium via nitrite to nitrate, which the plants partially absorb for growth; but usually accumulates steadily in the water. This nitrate is removed together with other substances that accumulate in the aquarium water over time through occasional partial water changes with low-nitrate water (tap water, rainwater).
Because of a possible occurrence of the nitrite peak, the stocking density of fish should only be increased carefully, even in aquariums that have been run in for a long time. Nitrification can be established more quickly in newly set up aquariums through vaccination bacteria from aquariums that have been run in. Users report on different experiences with corresponding "starter bacteria" from specialist dealers. This may be due to the fact that Nitrobacter added before the fish are stocked do not yet find a basis for life, disappear and thus remain ineffective. It is therefore advisable to monitor the nitrite concentration in the first few weeks after replenishing it with the help of an appropriate analysis set and, if the concentration is critical (0.5 mg / l are mentioned for many species), partial water changes should be carried out in order to keep below this mark to stay. In order not to hinder bacterial growth, the pelvic floor and filter should remain undisturbed during this phase.
In the event of a dangerously high nitrite concentration, which may have already led to the death of fish, it is advisable to use appropriate chemicals from specialist aquarists as a quick remedy in addition to a large partial water change. This is followed by inoculation with Nitrobacter, for example by inserting filter material that has been used for a long time from another aquarium into the filter of the affected tank, or with bacteria from specialist shops. In the following days, the nitrite concentration should be carefully monitored and, if necessary, the procedure should be repeated.