The nitrogen cycle in an aquarium, explained simply
Ammonia, nitrite, nitrate: understand what happens in a new aquarium, how long to wait and how to check it before adding animals.
In this guide
The nitrogen cycle is the transformation, by bacteria, of an aquarium's toxic waste into far less dangerous compounds: ammonia becomes nitrite, then nitrite becomes nitrate. In a new tank, these bacteria are absent or far too few. It generally takes four to six weeks for them to become established, which is why no fish or shrimp should be introduced before the end of this cycle, confirmed by testing.
Where does ammonia come from?
Anything that breaks down in the water releases nitrogen as ammonia and ammonium: animal waste, uneaten food, dead leaves, but also, at start-up, certain new nutrient-rich substrates. These two forms coexist. Un-ionised ammonia is roughly a hundred times more toxic to fish than ammonium, and its share increases as pH and temperature rise.
Even at low concentration, ammonia stresses fish and damages their gills; at higher doses, it kills. In an aquarium without established bacteria, it builds up without being removed.
The three stages of the cycle
Ammonia
At the start, ammonia is present and nothing yet transforms it. Nitrifying bacteria begin colonising every surface: filter media, substrate, decor, glass.
Nitrite
A first group of micro-organisms oxidises ammonia into nitrite. This was long attributed solely to bacteria of the genus Nitrosomonas; a study of 27 freshwater aquarium filters showed that archaea, other micro-organisms, were most often dominant there. For you, the principle doesn't change: ammonia levels fall, but nitrite levels climb. The nitrite peak often occurs around the third week. Nitrite is also highly toxic: it hinders oxygen transport in fish blood.
Nitrate
A second group, including bacteria of the genus Nitrospira, in turn converts nitrite into nitrate. Nitrate is far less toxic, but builds up over time. It is partly consumed by plants and removed during water changes.
When ammonia and nitrite stay at zero while waste continues to be produced, the cycle is established: both bacterial populations are numerous enough to keep pace.
How to start the cycle
An aquarium with no source of nitrogen won't cycle: the bacteria need food. Several approaches exist, all without animals:
- A pinch of food every two or three days, which breaks down and provides ammonia.
- Plenty of plants from day one, which supply organic matter and consume some of the nitrogen compounds.
- A little filter media from a healthy aquarium, which seeds the tank with bacteria.
- Bottled bacteria, which can help kick-start the process, without excusing you from checking with tests.
Keep the filter running at all times, day and night: the bacteria need a flow of oxygenated water. Treat tap water with a conditioner if it contains chlorine or chloramines.
Tracking the cycle with tests
Tests are the only way to know where your aquarium stands. Measure two to three times a week and record the results: you'll see an ammonia peak appear, then a nitrite peak, then nitrate settling in for good.
Multi-parameter strips let you track the trend quickly. Dennerle WaterTest 6in1 strips measure pH, KH, GH, nitrite, nitrate and chlorine, but not ammonia: during start-up, supplement them with a dedicated ammonia/ammonium test. For a precise value, a drop test remains more accurate than strips.
The cycle can be considered complete when:
- ammonia is at zero;
- nitrite is at zero;
- nitrate is measurable;
- and these results hold steady for several days running.
Then do a water change of around 30 to 50% to bring nitrate down, and introduce the animals gradually, in small numbers: the bacteria will adjust their population to the new load.
After the cycle: keeping the balance
The cycle never really stops. To keep it stable:
- change 10 to 30% of the water each week, depending on stocking and volume;
- rinse the filter media in water taken from the aquarium, never under the tap;
- don't replace all the filter media at once;
- feed in moderation, in a quantity eaten within a few minutes;
- remove decaying leaves and leftovers.
Nitrate should stay moderate. A value below 25 mg/L is a sensible target in a fish tank, and it's better to aim lower for shrimp, which are sensitive to waste build-up. If algae move in regardless, our guide to algae in the first few weeks helps identify the cause.
Common mistakes
- Adding animals after just a few days: this is the most common cause of losses in a new tank.
- Trusting clear water: clear water can still contain toxic levels of nitrite.
- Cleaning everything at once: filter, substrate and decor washed the same day, and the bacteria start again from almost nothing.
- Turning the filter off at night: without water flow, the bacteria suffocate.
- Overstocking all at once: even a mature tank needs time to absorb a sharp rise in waste.
Going further
The nitrogen cycle is the first step in any aquatic project. You'll find it at the heart of our guide to starting out in aquascaping and our comparison of which aquarium for freshwater shrimp. The My first aquascape project also details the timeline of the first few weeks, and our guide to a 30-litre nano aquascape applies it to a small tank.
Sources
Frequently asked questions
How long does the nitrogen cycle take in an aquarium?
Generally four to six weeks, sometimes a little less with filter media from a healthy tank, sometimes more in cold water. Only repeated testing can confirm it's finished.
How do I know when the nitrogen cycle is finished?
The cycle is established once ammonia and nitrite stay at zero for several consecutive days, while nitrate is measurable. A water change then lets you bring nitrate down before introducing the first animals.
Are bottled bacteria enough to skip the cycle?
They can help it start faster, but they don't replace testing. Keep testing the water and only introduce animals once ammonia and nitrite are reliably at zero.
