Nitrogen Cycle
The nitrogen cycle (N-Cycle) may seem complicated, but it’s really not. In short, we are using one of Mother Nature’s tricks to stay clean and safe by growing bacteria so our tanks can stay healthy for our fish. In our aquariums, we rely on the N-Cycle because this ecosystem is made up of beneficial bacteria (BB), which our fish literally depend on to survive and thrive in these closed systems. Remember, a “Freshwater Naturalist” wants to mimic the same processes found in nature. Pachamama (Mother Nature) has already figured out how to naturally keep a toxic environment clean.
A good portion of our hobby is literally dedicated to the well-being, care, and attention required to culture bacteria! This graphic explains the mechanisms involved in the N-Cycle.
The Toxic Player #1: Ammonia (NH₃)
Ammonia contains one nitrogen atom and three hydrogen atoms. This nitrogen-based chemical results from detritus, fish waste (poo and pee), and decaying material from plants and animals. Specific groups of beneficial bacteria process ammonia into something less toxic called nitrites. During the first phase of nitrification, the hydrogen is utilized, while the nitrogen combines with oxygen to yield NO₂⁻ (nitrites).
The Hero: Nitrosomonas europaea bacteria – These bacteria consume ammonia but can only do so in the presence of oxygen. The more oxygen available, the more efficiently these bacteria process ammonia. They can double their colony size in less than 12 hours.
The Toxic Player #2: Nitrites (NO₂⁻)
Nitrites contain one nitrogen atom and two oxygen atoms. This is no longer ammonia—it is now a completely different chemical compound, though it retains the original nitrogen atom. A different species of beneficial bacteria processes nitrites and converts them into nitrates by combining NO₂⁻ with one more oxygen molecule.
The Hero: Nitrobacter winogradskyi bacteria – These bacteria consume nitrites but, like Nitrosomonas, they require oxygen to do their job. In the chemical transformation from nitrites (NO₂⁻) to nitrates (NO₃⁻), an additional oxygen atom is added as a result of Nitrobacter bacteria doing their work in our aquariums.
Nitrates (NO₃⁻)
Nitrates contain one nitrogen atom, with beneficial bacteria adding an additional oxygen atom during their metabolism. Nitrates are the least toxic form of the three compounds discussed here. There is no process in the aquarium to take nitrates further into another compound.
In planted tanks, some plants process nitrates as food, making it excellent fertilizer for your garden. Instead of throwing your water change water down the sink, use it to water your garden! Ideally, aquarists should perform enough water changes to keep nitrate levels below 40 ppm.
Nitrospira – The Unsung Hero
Nitrospira, often pictured in pink bio-sludge, is another big player in nitrification. Until recently, it was believed that Nitrospira bacteria only worked against nitrites. However, research published in November 2015 revealed that Nitrospira bacteria also play a role in the nitrification of both ammonia and nitrites.
This evidence, combined with older research, suggests that our tanks may not only rely on Nitrosomonas and Nitrobacter species but also on a robust ecosystem including Nitrospira. What does this mean for the common aquarist? Not much, other than confirming that our aquariums depend on beneficial bacteria ecosystems. Regardless of the exact species responsible, the focus should remain on creating an environment where these bacteria can thrive.
Great filtration, quality filter media, and good husbandry ensure that beneficial bacteria of all kinds can do their job effectively.
Where Does This Happen in the Tank?
In aquariums, the filter is the primary location where bacteria establish colonies. Ceramic noodles, volcanic rocks, and other biomedia provide excellent surfaces for BB to colonize and multiply. Filters also promote water movement, which translates to good aeration—essential since oxygen is crucial for the bacteria to metabolize ammonia and nitrites.
Filters are also where nutrients like ammonia and nitrites accumulate, as organic matter decomposes. These three factors—nutrition, oxygen, and a physical space to grow—are abundant in filters, making them the best place for BB colonies to thrive.
Never fear changing too much water due to concerns about losing beneficial bacteria. This is a myth. Any loose bacteria in the water column will eventually resettle in your filter’s biomedia. You can also seed a tank by squeezing the muck from cured filter media into your new tank’s water column. The new tank’s filters will process this muck, catch the bacteria, and provide them with a suitable environment to establish colonies.