The Nitrogen cycle is rather simple but a vital biological process in aquarium keeping. To break it down, it is a biochemical cascade with three(3) stages involving continual chemical degradation of nitrogenous compounds [ammonia (toxic) to nitrite (toxic) to nitrate (non-toxic)].

As an enthusiast of this field, you’d have probably heard of terms like Nitrification Process, Biological Cycle or New Tank Syndrome being thrown around, but they all simply mean “THE NITROGEN CYCLE”.

STAGE 1: Ammonia (NH3 ) / (NH4 ) content in water

 Ammonia is a highly toxic nitrogen compound released from the plant’s waste and fish’s excrement (urea) which could possibly kill the fish or other livestock. Yes, fish can be killed by their own wastes as ammonia can literally burn their gills and skin.

Depending on the pH levels in your aquarium;

  • 7 & below produces ionized ammonium (NH4).
  • Above 7 produces ammonia (NH3).

(both are equally harmful)

On the other hand, the formation of ammonia is also contributed by the decaying uneaten food in the aquarium and unclean water.

Ammonia will continue to build up until the bacteria that eat it starts to form. Your tank may seem cloudy when these bacteria are starting to form. A sudden drop in the spiked up amount of ammonia indicates the beginning of stage 2.


  • Do not introduce too much livestock at once. [production of ammonia is lesser]

STAGE 2: Conversion of ammonia (NH3 ) / (NH4 ) into Nitrite (NO2 )

Now, thankfully, the automatically formed nitrifying bacteria in the water which is called as Nitrosomonas, partially oxidises ammonia into nitrite (NO2 ). In simple words, these bacteria eat ammonia or ammonium and release nitrite. Nitrite content in your tank is equally alarming as it is highly toxic for both fish and plants.

Nitrite is poisonous as it actively binds with haemoglobin in fish’s blood cells to form methaemoglobin instead of letting oxygen molecules bind and form oxyhaemoglobin. In simple words, nitrite hijacks the fish’s bodies hence, they suffocate to death eventually due to lack of oxygen in their bloodstream. Excessive supply of oxygen (O2 ) into the tank will NOT make one-bit changes to this situation. 

Just like the first stage, nitrites build up until a colony of bacteria develops to dispose of them and welcomes stage 3.


  • Buy a bottle of live nitrifying bacteria or get some used filter media from a friend (it may be dirty but sure has lots of beneficial bacteria on them) to speed up this whole process so, you can avoid the dangerous spike up in ammonia level. [conversion of ammonia to nitrite happens vigorously]

STAGE 3: Conversion of Nitrite (NO2 )  into Nitrate (NO3 )

This conversion happens when nitrite-loving bacteria called the Nitrobacteria fully oxidises nitrite into nitrate. In simple words, nitrite is eaten by these bacteria and released as nitrate into the water. Nitrate is the end product of the Nitrogen Cycle and it is NOT toxic.

During this period, the levels of ammonia and nitrite (the toxic nitrogenous compounds) will reach 0ppm (parts per million) which means the tank has been cycled successfully! Nitrate is useful absorbent of plants from the soil to build up protein.

Now that the tank is actively involved in these conversions, resulting in nitrate formation at the end, is all good but does not mean the work is done.

Nitrate (NO3 ) is harmless to fish and useful to aquatic plants in small amounts only. With that being said, increased concentration of nitrate in water, for instance, above 20ppm (depending on the aquatic species in the tank) even after the absorption by plants, can turn into a toxic compound hence, regular monitoring on the levels is crucial.

However, regular maintenance job and the introduction of new aquatic plants should be enough to keep this level at bay as plants consume nitrate for protein build up. The frequency of water changing is directly proportional to the number of livestock in the tank.


  • Change the water regularly and keep adding plants to the aquarium. [elimination of excessive nitrate takes place regularly].

Get NH3 NH4, NO2, NOTest Kits to constantly monitor the levels of these compounds in your tank. Possession of these kits is compulsory.


Image 2 shows a the nitrogen cycle time line
Image 3 shows an example of a graph of nitrogenous compounds’ levels in a tank

REFERENCES: 2021. Nitrogen Cycle | Petco. [online] [Accessed 31 March 2021].

Adams, C., 2021. The Nitrogen Cycle: Simple Step by Step Guide For Beginners. [online] Modest Fish. [Accessed 31 March 2021].