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  • Writer's pictureStewart Kessel

Aquarium Science: The Nitrogen Cycle

Updated: Jan 8

What goes on in your aquarium filter and how to avoid new tank syndrome?

Water quality is fundamental to keeping your fish happy and healthy. Poor water-quality will stress your fish, increase the likelihood of diseases, shortening life expectancy and ultimately will kill your fish. It is often suggested ‘fishkeeping is more about keeping water than fish’.

Like other living organisms, fish as part of their biological processes excrete waste. Typically, this will include ammonia through their gills and in their urine. Ammonia is toxic to fish and will burn their skin and cause irritation to the gills leading to breathing issues. In the natural world, this toxic waste is significantly diluted by the environment, but in the confines of an aquarium, the ammonia will rapidly build-up to harmful levels.

Fortunately, nature has an answer in the form of the ‘Nitrogen Cycle’ which offers a method to eliminate ammonia.

The Nitrogen Cycle

The nitrogen cycle is a three-stage process, whereby ‘friendly’ aquarium bacteria first convert the toxic ammonia into toxic nitrite and then into the less toxic nitrate.

As shown in the diagram, there are two general types of beneficial nitrifying bacteria required for the nitrogen cycle. Firstly, Nitrosomonas species that use ammonia as a nutrient source to produce nitrite, and secondly Nitrobacter species that in turn use the nitrite nutrient source to produce less harmful nitrate.

For these beneficial bacteria to establish, they require somewhere to anchor, e.g. filter media, aquarium substrate or other surfaces. They also need the right conditions, e.g. temperature, pH, oxygen and their specific source of nutrients.

An established aquarium has a mini-ecosystem in which the fish, other livestock and plants, through the nitrogen cycle, form a stable environment that with routine maintenance is easily sustained.

A New Tank

However, this is not true in a newly set-up aquarium, where there are initially insufficient beneficial bacteria, in the substrate and filter media, to convert the toxic ammonia and nitrite to the less toxic nitrate.

The Nitrosomonas species bacteria once given a source of ammonia will start to multiply and colonise the filter and substrate. However, these bacteria are slow to grow and will need time to reproduce to the point of being able to remove enough ammonia for fish to be safely added. Further to this, the Nitrobacter species bacteria will not start to colonise and multiply until there is a sufficient source of nitrite. Again, due to their slow reproduction, not all the toxic nitrite will be converted into less toxic nitrate.

Therefore, if the livestock is added too soon to a newly set-up aquarium, issues will arise very quickly, leading to the loss of fish, often referred to as ‘New Tank Syndrome’.

Slowly adding a few hardy fish to help establish an aquarium was regularly advocated in the past. However, for the reasons stated at the beginning of the article, this is no longer the preferred or accepted method.

Therefore, when setting up a new aquarium, it is best to complete what is termed a ‘Fishless tank cycle’ before adding your chosen fish.

A fishless tank cycle is a technique used to establish the bacterial colonies required for the nitrogen cycle without the presence of fish, hence the term fishless. This process involves getting the nitrogen cycle running by introducing a source of ammonia into the newly set-up and virtually sterile aquarium. With no livestock, the presence of ammonia is not an issue, and with time the nitrifying bacteria will multiply and colonise the filter.

Beneficial bacteria are prevalent in most sources of water. To boost their presence and help start the process to colonise your aquarium filter, you can either use some media from another established filter or use commercially available bottled bacteria.

There are various techniques to introduce ammonia, the most reliable method is to add liquid ammonia direct, rather than, for example, adding food and allowing it to rot.

The ammonia will enable the Nitrosomonas species bacteria to establish and convert the ammonia to nitrite. At which point the Nitrobacter species bacteria will start to colonise the filter, converting the nitrite to nitrate.

Continually dosing the ammonia during the cycling process will feed the cycle until such a point when it is evident that the cycle is complete. At this time the dosed ammonia and resultant nitrite will both fall to zero within a 24-hour period, and the nitrate level will significantly increase.

At this point, dosing of ammonia can stop, and a water change is required to reduce the nitrate level. Addition of your chosen fish and other livestock can then follow, whose waste will maintain the cycle.

For a more detailed step-by-step guide to Fishless Tank Cycling see our dedicated blog on the subject.

Author: Stewart Kessel CChem, MRSC


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