Nitrogen Cycle

The Nitrogen Cycle


The Nitrogen cycle is the most important biochemical process to occur in your marine aquarium.


It is carried out by microscopic bacteria found in all seawater and occurs naturally in the ocean, understanding it is our key to successful marine aquarium keeping.


This cycle is the cornerstone of biological filtration and is crucial to initiate correctly when you first set up your aquarium, this is called “cycling”.


Here’s how it all works:


Waste is excreted from marine life in the form of protein, which is quickly mineralized into ammonia/ammonium (NH3 or NH4). Ammonia also comes from the breakdown of food particles and decaying organisms.


This ammonia is highly toxic to marine life, but luckily nature has a solution for us in the form of a marine bacterium: Nitrosomonas.


Nitrosomonas use this substance as their energy source and aerobically convert it to nitrite (NO2). Nitrite is less toxic but still nasty.


Then another bacterium (Nitrobacter) comes along and converts nitrite to nitrate (NO3).


Nitrate is also harmful to marine life in high concentrations; as little as 20ppm will harm corals (however corals require trace levels of nitrate to grow well) and fish will become uncomfortable when levels reach 50ppm.


It is these 2 types of bacteria that make up our biological filters by growing on live rock, sponges or other types of filter media.


The final phase of the nitrogen cycle is called denitrification.


This is an anaerobic (oxygen free) reaction whereby anaerobic bacteria covert the nitrate into harmless nitrogen gas that bubble to the surface of the water and enters the atmosphere completing the nitrogen cycle.


Another way to get rid of climbing levels of nitrates other than by using live rock is by doing regular partial water changes, growing marine plants which use nitrate for growth or by using a denitrifying filter.


While cycling your aquarium, ammonia and nitrite levels tend to peak and then drop to undetectable levels as they are converted to the next compound in the pathway, but unless the nitrate is removed manually or you have enough live rock to cope with this it will soon build up to harmful levels.


Cycling is initiated by an ammonia source such as decaying matter from live rock or a commercial ammonia product.


You’ll use your ammonium, nitrite and nitrate test kits to establish which phase the aquarium is in. It will go from ammonium spike to nitrite spike to undetectable levels of ammonium/nitrite and detectable nitrate.


The entire cycling process will take anywhere from 3-100 days depending on what type of biological filtration you are using: a commercial filter and/or cured or uncured live rock.


The only way to tell which phase the aquarium is in is to detect zero nitrite over time and to observe various parameters such as pH start to stabilize.


Cycling should be completely finished by the time livestock is added to an aquarium.


Adding your marine life needs to happen gradually so the different bacterial populations have a chance to adjust to the increasing bioload. A mistake here could easily cause the whole system to crash.


Having a correctly functioning nitrogen cycle in your aquarium requires regular water testing and ensuring that the bacterial populations in your biological filter are not physically (such as any process that removes to much from the filter media) or chemically (adding something to the water that will kill them off) damaged or removed.


Up until relatively recently the nitrogen cycle was not fully understood in saltwater aquariums which resulted in high mortality for captive marine life.


Now that we understand it fully this invisible process allows us to keep an amazing array of dazzling marine creatures in perfect health in our homes.