Guide
The Aquarium Nitrogen Cycle: Ammonia, Nitrite, Nitrate
Every healthy aquarium depends on bacteria you can't see. Here's what they do, how long they take to establish, and how to avoid knocking them back.
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The cycle in three steps
- Ammonia. Fish excrete ammonia through their gills, and uneaten food and decaying matter add more. UF/IFAS notes that the more fish are fed, the more ammonia they produce, though even starved fish produce some.
- Nitrite. One group of nitrifying bacteria converts ammonia to nitrite. Nitrite is also toxic: UF/IFAS gives a level as low as 0.10 mg/L as harmful to fish.
- Nitrate. A second group converts nitrite to nitrate, which is much less toxic. Nitrate accumulates in a closed aquarium until water changes or plants remove it.
This bacterial process is nitrification. UF/IFAS describes it as aerobic: the bacteria need oxygen.
How long it takes
UF/IFAS says a new biofilter requires six to eight weeks to build up sufficient bacteria to effectively reduce ammonia and nitrite. During that time, ammonia rises first, then nitrite as the first bacteria establish, then both fall as nitrate appears.
A tank is generally considered cycled when ammonia and nitrite both read zero and nitrate is rising, with the normal bioload in place. Test with your own kit rather than going by time alone.
New tank syndrome
Adding many fish to a new tank before the biofilter is ready lets ammonia and nitrite build up. UF/IFAS calls this new tank syndrome. Common ways to reduce the risk are stocking slowly, feeding lightly at first, and testing ammonia and nitrite frequently so you can do water changes when levels rise.
What the bacteria need
- Oxygen. Nitrification needs it. UF/IFAS advises taking steps to increase oxygen if it falls below 5 mg/L.
- Alkalinity. Nitrification consumes alkalinity and produces acid. UF/IFAS says common nitrifying bacteria don't function effectively below a total alkalinity of 20 mg/L, and that once alkalinity is depleted, pH can drop significantly and quickly, harming fish and bacteria.
- Surface area. Bacteria grow on filter media, gravel and other surfaces.
- No chlorine or chloramine. EPA notes that chlorine and monochloramine can prevent the growth of beneficial bacteria, which is one reason tap water must be treated before it goes in.
Things that set the cycle back
- Rinsing filter media under chlorinated tap water.
- Replacing all filter media at once.
- Long filter outages, such as a power cut, which starve bacteria of oxygen.
- Some medications.
- Very low alkalinity or a pH crash.
- A sudden large increase in fish or food.
Living with the cycle
Once established, the cycle keeps ammonia and nitrite at zero but keeps producing nitrate. Regular testing and water changes keep nitrate down; see the water change calculator. If ammonia ever shows up in an established tank, use the ammonia calculator to see how much is in the toxic form, and look for the cause.
Testing during cycling
Test ammonia and nitrite every day or two during the first weeks, plus pH, and nitrate once nitrite starts falling. UF/IFAS stresses testing immediately whenever fish behavior or appearance changes. Our testing guide covers what to test and how.
What test results look like as a tank cycles
The order of events follows from the two groups of bacteria establishing one after the other:
| Stage | Ammonia | Nitrite | Nitrate |
|---|---|---|---|
| Start | Rising | Zero | Zero |
| First bacteria establishing | Peaks, then falls | Rising | Low |
| Second bacteria establishing | Low or zero | Peaks, then falls | Rising |
| Cycled | Zero | Zero | Rising between water changes |
If ammonia or nitrite climbs to levels that worry you while fish are in the tank, a water change with conditioned water lowers it immediately without stopping the cycle.
Where the bacteria live
Most nitrifying bacteria live attached to surfaces, not floating in the water. Filter media with lots of surface area hosts most of them, with more on gravel, rock and decorations. That's why filter care matters more than the water itself for keeping the cycle, and why a water change, even a large one, doesn't remove the biofilter.
Common questions
How long does it take to cycle a new tank?
UF/IFAS says a new biofilter requires six to eight weeks to build up enough bacteria to reduce ammonia and nitrite effectively. Test regularly rather than relying on the calendar.
What is new tank syndrome?
Fish deaths in a new tank caused by ammonia and nitrite building up before the biofilter has established. It's a common problem when many fish are added to a new tank at once.
Can I clean my filter?
Yes, but gently. The bacteria live on filter media and surfaces. Rinse media in removed tank water rather than chlorinated tap water, and avoid replacing all media at once.
Do medications affect the cycle?
Some can. Read the label for effects on beneficial bacteria, and test ammonia and nitrite during and after treatment.
Sources
- University of Florida IFAS Extension, Ammonia in Aquatic Systems (FA031)
- U.S. EPA, monochloramine Q&A on aquariums
Checked Nov 11, 2026. Spot an error? Tell us.