The nitrogen cycle that occurs in an aquarium tank is a small replica of what happens in the wild. Because the aquarium does not have all the elements of the wild, human intervention is required to maintain the ecosystem balance.
The Nitrogen Cycle Starts With The Fish
The nitrogen cycle starts with the introduction of fish in the aquarium tank and food. As the fish eats the food and uses the oxygen in the water to produce energy, the fish also produces waste material.
The Nitrogen Cycle
The waste material is made up of carbon dioxide and fecal matter. The fecal matter decomposes into nitrogen compounds such as ammonia. In order to maintain the delicate balance, the nitrogen and carbon dioxide must be removed somehow.
The carbon dioxide is mainly removed through 2 processes. The first is aeration or the contact between the water surface and air. Carbon dioxide is exuded into the air and oxygen mixes into the water. If you have live plants, they use carbon dioxide and nitrates (explained below) in photosynthesis and produce oxygen as their waste material.
Bacteria Break Down Ammonia And Nitrites
The ammonia is mainly reduced through bacteria called nitrifiers that reside in the sand, plant leaves, gravel, filter systems, or live rocks. One type of nitrifier (called nitrosomonas) converts ammonia into nitrite. Another type of nitrifier (called nitrobacter) converts nitrite to nitrate.
The first type of bacteria is introduced as soon as ammonia is present. The second type of bacteria is introduced as soon as nitrites are present.
Although all 3 nitrogen compounds are toxic to fishes (ammonia, nitrite, and nitrate), nitrate is significantly less toxic than the others. However, you still want to maintain your nitrate level down below 10 ppm (parts per million). In the wild, the nitrate levels are usually below 5 ppm.
Nitrate are the primary food for algae so if your tank has an algae problem, this usually indicates you have high nitrate levels. Some people purposely introduce macroalgae to temporarily lower nitrate levels.
In order for nitrifiers to convert nitrites to nitrates, they need an abundant source of oxygen (for a biochemical process called oxidation). However, if there’s low levels of oxygen then the reverse happens. The bacteria convert nitrates back to nitrites.
However, unless there’s a sudden increase in bacteria growth (called a bacteria bloom), oxygen levels are not normally a concern unless you’re overstocking your tank.
And finally, the nitrate is removed by your:
- Filtration system
- Changing water
- Live organisms that use photosynthesis (i.e. plants or corals)
Keeping Your Nitrifiers Happy
The bacteria in this cycle like having certain environments. The first is a stable pH level that doesn’t fluctuate too wildly, preferably between 7 – 8.5.
The nitrifiers also like a temperature range between 65 – 85. Again, reducing temperature fluctuations is more important than just keeping your temperature within this range.
Salinity (or salt conent of the water) is of little concern to the bacteria but they are sensitive to UV radiation. Fortunately, glass naturally screens out UV but acrylic does not.
Because of their light sensitivity, a favorite breeding ground of these bacteria is in the filtration system. They often form slimy colonies and healthy colonies have an earthy smell.
Starting The Nitrogen Cycle
There’s an entire section on starting the nitrogen cycle but one thing you do not want to do is start it with live fish. Most fish cannot endure the ammonia and nitrites while bacteria colonies develop.
Remember, nitrosomonas (converts ammonia to nitrites) only develop in the presence of ammonia. While they develop and build up their numbers, the fish is already being stressed by the presence of ammonia.
Then when the ammonia is finally starting to convert to nitrites, only then do the nitrosomonas start to develop. By the time enough nitrosomonas colonize and convert nitrites to nitrate, the fish would probably not have survived.