The Aquarium Nitrogen Cycle

How beneficial bacteria convert toxic ammonia to nitrite and then nitrate, and why establishing this cycle matters before stocking fish.

What it is

The nitrogen cycle is the biological process that converts nitrogenous waste into progressively less harmful compounds. In an aquarium it follows the sequence ammonia (NH3/NH4+) to nitrite (NO2-) to nitrate (NO3-). This conversion is carried out by nitrifying bacteria that colonise filter media and other surfaces. Without this microbial population, waste accumulates and the water becomes toxic to fish.

Where ammonia comes from

Ammonia enters the water from fish excretion, uneaten food, and decaying organic matter. The decomposition of organic nitrogen into ammonia is called ammonification. Ammonia exists in two forms: un-ionised NH3 and ionised ammonium NH4+. Un-ionised ammonia is far more toxic to fish than ammonium, and its proportion rises with higher pH and temperature.

How nitrification works

Nitrification is an aerobic, two-step oxidation. First, ammonia-oxidising bacteria such as Nitrosomonas convert ammonia to nitrite. Second, nitrite-oxidising bacteria such as Nitrobacter and Nitrospira convert nitrite to nitrate. Some Nitrospira (comammox) can perform both steps within a single organism. The overall first step can be written 2NH4+ + 3O2 to 2NO2- + 4H+ + 2H2O, followed by 2NO2- + O2 to 2NO3-.

The beneficial bacteria

These bacteria are autotrophic and slow-growing. They attach to surfaces with good water flow and oxygen, primarily the filter media, but also substrate, decor, and tank walls. Because nitrification is aerobic, the bacteria require dissolved oxygen, and they grow fastest near neutral to slightly alkaline pH and warm temperatures.

Why cycling matters

Until the bacterial colonies are large enough to process the daily waste load, ammonia and nitrite accumulate to levels that damage fish gills and tissues. Establishing the cycle (commonly called cycling) before adding a full stock of fish is what protects them. A biofilter typically needs several weeks to mature.

What happens to nitrate

Nitrate is the end product of nitrification and is far less toxic than ammonia or nitrite, but it still accumulates over time. It is removed by routine water changes, uptake by live plants, and, in anaerobic zones, by denitrification, which reduces nitrate back to nitrogen gas.