The nitrogen cycle is the conversion of the essential element nitrogen into chemicals, amino acids, and proteins. Nitrogen is bound up inside the bodies of organisms but is released back into the water and air over time. Establishing the aquarium nitrogen cycle is one of the first steps to preparing a fish tank to hold pets.
Nitrogen Cycle Aquarium Importance
Without cycling, ammonia and other toxic compounds build up until your fish die. Nitrogen is important for the formation of proteins. All animals create proteins as they grow but they also break them down.
The continual breakdown of proteins releases simple amino acids and ammonia as waste. Bacteria in your aquarium water also feed on protein sources like leftover fish food. Releasing ammonia in the process.
Ammonia is NH3+, so it contains nitrogen. But animals can’t make use of it as a nitrogen source. In the wild, ammonia never builds up to toxic levels. But in a fish tank the water volume is much smaller. Ammonia levels may become fatal in a few days if your tank is crowded and the nitrogen cycle is not established.
The Aquarium Nitrogen Cycle Explained
The nitrogen cycle in water is managed through the action of microorganisms. These nitrifying bacteria are from several groups, including the genera Nitrosomonas and Nitrobacter.
Each stage of the process involves a separate group that feeds on one compound. They re-release nitrogen, transformed into a reduced form that is food for yet another group of organisms. The process is important for the health of Earth’s ecosystem.
Ammonia in the Nitrogen Cycle
The first step is the transformation of free ammonia (NH3+). Ammonia is the preferred source of nitrogen for plants as fertilizer as it is energy-rich and easy to convert back into proteins. But animals find it poisonous. No level of ammonia is considered safe for an aquarium.
Ammonia toxicity is worse when the pH of an aquarium is high. Signs of ammonia poisoning include gasping for air, bright red or purple gills, lack of appetite and lethargy. Some fish will even show ammonia burns; bright red or black patches on their skin.
Ammonia to Nitrite
No concentration of ammonia is safe for freshwater and marine life. Nitrosomonas and its relatives are the first stage in the aquarium nitrogen cycle. They feed on ammonia, converting its nitrogen into nitrite (NO2-).
Nitrite is not as dangerous to fish as ammonia. Concentrations of 0.1-2.0 parts per million are tolerable for most creatures. But detectable nitrite levels are still a sign that some part of your nitrogen cycle isn’t working. Water changes or the addition of aquarium salt will detoxify free nitrite in your aquarium water.
Where Does the Nitrate Cycle End?
Assuming your water nitrogen cycle is intact, Nitrobacter and its cousins feed on nitrite. They convert it into nitrate (NO3-). Nitrate is far less toxic than either ammonia or nitrite.
Fish will show no ill effects at levels of 10 to 30 ppm, depending on the species. Even sensitive marine corals should have 0.5-10 ppm of nitrate since their symbiotic algae (zooxanthellae) need it as a food source.
In most fish tanks, nitrate levels will rise until you perform a water change. Nitrate is the end stage in most tanks but not in nature.
There are two places for nitrate to go. The first is into fueling the growth of aquatic plants. Plants prefer ammonia first, then nitrite, and then nitrate if no other fertilizer is available. Lush live plant growth is even used as a replacement for a filter in Walstad aquariums, which use bioactive principles to mimic natural ecosystems.
The second way to flush nitrate from the system is through the activity of denitrifying bacteria. These microorganisms feed on nitrate and convert it into nitrogen gas (N2). Nitrogen gas then floats back into the atmosphere. Completing its long journey from air molecule to amino acid to protein to ammonia to nitrite to nitrate to air molecule.
Denitrifying bacteria are not an important step in the fish tank nitrogen cycle, however. They are anaerobic (oxygen-hating) bacteria that are poisoned in well aerated environments. In nature you find anaerobic microorganisms deep in the mud, under rocks, in decaying piles of organic matter…All places where oxygen levels remain low.
In an aquarium we want the tank to be as well oxygenated as possible. Otherwise anaerobic decay will make smelly and poisonous chemicals like hydrogen sulfide. So we have to handle nitrate by adding plants or removing it via water changes.
Fish Nitrogen Cycle and New Tank Syndrome
The nitrogen cycle is why new tank syndrome is a problem when first starting an aquarium. New tank syndrome happens when fish die after being added to a tank.
Aquarists unfamiliar with the nitrogen cycle keep adding fish, which then keep dying. The problem is that the beneficial bacteria that maintain the aquarium nitrogen cycle need time to grow.
If you are slow cycling an aquarium, the tank will take 1 to 3 months to reach maximum carrying capacity. There are ways to jump-start the process, however. Using bottled nitrifying bacteria is the easiest way to cycle a tank.
Other aquarists prefer fishless cycling using pure ammonia or adding nitrogen cycling bacteria from another established tank.
Conclusion
A fish tank ecosystem needs microorganisms to prevent the fatal buildup of nitrogenous waste. Ammonia, nitrite, and nitrate are reduced and removed through the aquarium nitrogen cycle. A process that all aquarists need to understand to preserve fish health.
FAQs
Fish poop and other waste is broken down by bacteria and fungi in your tank. Snails and other invertebrates will also eat some of it but make poop of their own. Fish waste does not disappear forever. It decays into ammonia and other compounds that your beneficial nitrifying bacteria detoxify.
The aquarium nitrogen cycle happens everywhere, including in your fish tank water. But the bacteria live in the greatest numbers in your filter and substrate. They prefer hard surfaces with lots of surface area, access to oxygen, and ample water flow. Filter floss, ceramic bio balls, and lava rock are valuable filter additions for this reason.
An aquarium takes anywhere from a week to a couple of months to cycle. The fastest way to cycle a new tank is to add bottled beneficial bacteria. Otherwise, the organisms exist everywhere, including in the air. They will find their way into an aquarium over time and multiply as they feed on ammonia and nitrite.
