The Nitrogen Cycle

By: Joe S.

When first starting an aquarium, one must take into account the increasing levels of toxicity in the water. The three major toxins that the aquarist must test and control are ammonia, nitrite, and nitrate. Ammonia being the most toxic is comprised of two elements: nitrogen, and hydrogen (NH3). In waters with a low pH (more free protons, H+) the ammonia is readily altered to form ammonium (NH4+), a non-toxic substance. In waters with a high pH (more free hydroxide ions, OH-) the ammonium is readily altered to form ammonia. Both ammonia, and ammonium naturally maintain a balance in the aquarium however in waters with higher pH values much of the ammonium is converted to ammonia due to the lack of free protons in the water, therefore the possibility of toxemia is more prevalent. Most test kits are made to test the “total ammonia” in the water, which is both the ammonia, and ammonium content in the water measured in mg/L. A reading of less than 0.1mg/L can be toxic to most aquatic animals.

U nitrogennder the proper conditions, bacteria known as Nitrosomonas oxidize the ammonia in the water, and remove the hydrogen atoms to form nitrite (NO2). These proper conditions consist of a pH of 7.2 to 8.5, a temperature of 68° to 86°F, and a very high oxygen content, any deviation from these parameters will lead to slow division, or in more radical cases, death of the bacterium. Nitrite is also very toxic to animals in the aquarium and unlike ammonia the level of pH cannot diminish its level of toxicity. Nitrite is toxic to all animals at concentrations less than 5mg/L. The nature of the compound allows it to readily form strong bonds with the hemoglobin in the blood of the fish. This is known as methemoglobinemia, and it can be identified by rapid movement of the operculum in an attempt to oxygenate the blood. If the fish does manage to recover from this, it will most likely result in some organ damage.

The parameters for optimal division of Nitrosomonas also hold true for a second type of oxidizing, or nitrifying bacteria called Nitrobacter. Nitrobacter consumes the nitrite and by oxidation, forms Nitrate (NO3). This substance is much less toxic to aquatic fauna, and is the desired byproduct in most aquaria housing less delicate species of fishes. High concentrations of Nitrate can even be beneficial to plants, tridacnid clams, macro algae, and some soft corals, however it can be highly toxic to many other invertebrates and stressful to some fish. There are few ways to efficiently filter this substance out of the water, but it can be diluted with regular water changes, or in the case of freshwater fishes, a type of water-softening mineral known as clinoptilolite, or more commonly as zeolite, can be used. There are some resins made for marine aquaria that are fairly efficient for it’s removal.

The final process in the nitrogen cycle is known as denitrification. This is the process in which nitrate is broken down into pure nitrogen by the removal of it’s oxygen atoms. Denitrification can take place only in oxygen free environments. There are special filters that allow for the removal of nitrate, i.e. the coil denitrator, the plenum, and a chemical denitrator. Some common species of bacteria that grow within these filters are Denitrobacillus, Micrococcus, Thiobacillus, Pseudomonas, and Sulfomonas. These bacteria are capable of growing in both aerobic, and anaerobic conditions. The only downside to these bacteria is that they produce gasses that can be toxic to the fishes and invertebrates. Proper care must be taken to make sure these compounds do not build up in the tank.

cowSpecific estimated times for the cycle of the aquarium can be found on the corresponding charts. The average length of time to cycle an aquarium is approximately 28 days, however poor conditions i.e. low temperature, or low pH can lead to an extended cycle period. Also, overloading, overfeeding, or adding new fish will disrupt and extend the cycle. One must remember that when medicating with antibiotics they will surely kill the beneficial bacteria in the aquarium. Also by removing too much water at one time, or changing the water and the filter at the same time one could deplete the bacterial colony. – Joe S.

Copyright 2013 Absolutely Fish, Inc. All Rights Reserved