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The nitrogen cycle

By the koi.wiki editorial team · Updated

Key facts: The nitrogen cycle in a koi pond is the biological process by which toxic ammonia excreted by fish and produced by decaying waste is converted, through nitrification, into nitrite and then into less harmful nitrate[1][2].
In nature, the nitrogen cycle is the broader biogeochemical cycle by which nitrogen moves between the atmosphere, land and water through fixation, ammonification, nitrification and denitrification[7]. In a koi pond, keepers are mainly concerned with one part of this cycle: the biological conversion of fish waste into progressively less toxic nitrogen compounds[1].Koi, like other fish, excrete nitrogen as ammonia, and uneaten food and other organic matter also decompose to release ammonia into the water[2][8]. Because ammonia is toxic even at low concentrations, pond keepers rely on nitrifying bacteria colonizing filter media and other wet surfaces to oxidize it into nitrite and then nitrate[1][4].

The three stages of nitrification

The nitrogen cycle in a pond proceeds in two main bacterial steps collectively called nitrification. First, ammonia (NH3) or ammonium (NH4+) is oxidized to nitrite (NO2–) by ammonia-oxidizing bacteria, commonly identified by the genus name Nitrosomonas, along with related genera such as Nitrosospira[1][4]. Second, nitrite is oxidized to nitrate (NO3–) by nitrite-oxidizing bacteria, most often identified as Nitrobacter, though Nitrospira is also cited as carrying out this step in some systems[1][5]. Nitrate is considered non-toxic at concentrations typically found in recirculating systems[4].According to the US importer Kodama Koi Farm, ammonia is consumed and converted to nitrite by Nitrosomonas, nitrite is then consumed and converted to nitrate by Nitrobacter, and nitrate can be removed from the pond by water changes or taken up by aquatic plants and algae[2]. The University of Florida IFAS Extension similarly describes ammonia-oxidizing bacteria (AOB) converting ammonia to nitrite, and nitrite-oxidizing bacteria (NOB) converting nitrite to the much less toxic nitrate[1].Nitrification is strictly an aerobic process, meaning it requires dissolved oxygen, and it also produces hydrogen ions, which can lower pH and consume alkalinity unless the water is adequately buffered[3][6]. Low dissolved oxygen limits the ability of nitrifying bacteria to convert ammonia and nitrite, so oxygen levels below 5 mg/L should be corrected[1].

Toxicity of ammonia and nitrite

Ammonia exists in water as un-ionized ammonia (NH3) and ionized ammonium (NH4+); the un-ionized form is approximately 100 times more toxic to fish and increases with higher pH and temperature[1]. Ammonia is colorless and odorless even at toxic levels, so testing is the only reliable way to detect it[1].Nitrite is also toxic; in fish it can bind with hemoglobin to form methemoglobin, which cannot carry oxygen, causing the fish to become starved for oxygen, with toxicity varying by species[6]. One documented mitigation is raising the chloride concentration of the water, since a chloride-to-nitrite-nitrogen ratio of about 10:1 helps protect against this toxicity[6]. Nitrate, by contrast, is considered far less toxic and is the stable end product of nitrification in most pond and aquarium systems[5][4].

Where nitrifying bacteria live

In ponds, nitrification occurs primarily on the surface layers of mud or substrate and on plants or other submerged structures where nitrifying bacteria colonize[1]. In tanks or aquaria, and in koi ponds relying on mechanical systems, a dedicated biological filter provides surface area for these bacteria to grow[1][9]. A newly started biofilter typically requires six to eight weeks to build up sufficient bacteria to effectively reduce ammonia and nitrite[1][4]. Readers can learn more about pond filtration and biological filter media used to support these bacterial colonies.Bacterial cells grow on nearly all wet surfaces in a system, including pipe interiors and tank walls, not only on dedicated filter media[4]. Nitrifying bacteria follow a continual cycle of growing, multiplying, maturing and dying, with cells regularly sloughing off the media and being replaced[4].

“New pond syndrome” and cycling a new system

When a biofilter has not yet matured, ammonia and nitrite can spike to dangerous levels because insufficient nitrifying bacteria are present to process waste; in aquarium keeping this is known as “new tank syndrome,” and the same underlying problem affects newly set up koi ponds[8]. According to Kodama Koi Farm, if a pond is new, the nitrogen cycle is not yet established, so it is important to seed the pond with beneficial bacteria, and even established ponds benefit from regular bacteria additions because the ecosystem changes daily as koi eat and produce waste[2].One method of starting a biofilter, called the “cold start method,” involves stocking fish without an already active biofilter; operators using this approach must be prepared for a rapid rise in ammonia and nitrite and may need water exchanges and reduced or suspended feeding until the biofilter activates[4]. Ammonia concentration typically rises to a peak and then declines, followed after a lag by a corresponding rise and decline in nitrite as nitrite-oxidizing bacteria establish themselves[4]. Alkalinity is also important during start-up: raising system alkalinity to around 150 mg/L supports Nitrosomonas growth, while establishing Nitrobacter has reportedly had better success at 200–250 mg/L, after which alkalinity can be allowed to decline to normal operating levels[4].Separately, Kodama Koi Farm discusses “old pond syndrome,” a condition it associates with apparently random koi deaths in established ponds, and recommends consulting a fish veterinarian or trusted koi dealer rather than attempting self-diagnosis[10]. For general approaches to spotting problems early, see signs of illness and diagnosis.

Responding to ammonia or nitrite spikes

If ammonia is detected, the University of Florida IFAS Extension recommends first reducing or stopping feeding for one to three days, since uneaten feed worsens the problem and most fish other than fry tolerate a short fast without harm[1]. Overfeeding is described as a major cause of high ammonia concentrations, and pausing feeding allows the nitrogen cycle to “catch up” with the nutrient load[1].

Testing and monitoring

Because ammonia is colorless and odorless, and nitrite and nitrate are likewise not detectable by sight or smell, regular water testing is necessary to track the three key nitrogen compounds[1][8]. Hobbyists commonly monitor ammonia, nitrite and nitrate levels, particularly during the early weeks of a new system, to confirm that the cycle is progressing as expected[8]. See test kits and monitors, and individual pages on ammonia, nitrite and nitrate for reference values and further detail.

Nitrate accumulation and removal

Unlike ammonia and nitrite, nitrate is not efficiently removed by the nitrification step itself and tends to accumulate over time in pond and aquarium systems unless actively managed[5]. In aquaria, the nitrogen cycle hobbyists refer to is not a true closed cycle because nitrogen is continually added through food and must be removed at the end via water changes, since most systems lack the bacteria needed to carry out full denitrification back to nitrogen gas[8]. Nitrate can also be taken up by live plants and algae as a nutrient[2][5]. Regular partial water changes remain the standard method for keeping nitrate from building up to harmful levels[5][1].

Common questions

What bacteria are responsible for the nitrogen cycle in a koi pond?
Ammonia-oxidizing bacteria, commonly identified as Nitrosomonas, convert ammonia to nitrite, and nitrite-oxidizing bacteria, commonly identified as Nitrobacter (with Nitrospira also cited in some sources), convert nitrite to nitrate[1][4][5].
How long does it take to establish the nitrogen cycle in a new pond or filter?
A new biofilter typically requires six to eight weeks to build up enough nitrifying bacteria to effectively reduce ammonia and nitrite[1][4].
Why is ammonia dangerous if it cannot be seen or smelled?
Ammonia is colorless and odorless even at levels toxic to fish, so testing water parameters is the only reliable way to detect it before it harms koi[1].
What should be done if ammonia or nitrite spikes in a koi pond?
Recommended steps include reducing or stopping feeding for one to three days, performing a partial water change where practical, checking and improving dissolved oxygen, and ensuring the biofilter is properly sized; for new ponds, seeding with beneficial bacteria is also suggested by a koi dealer source[1][2].
Does the nitrogen cycle remove nitrate as well as ammonia and nitrite?
Not automatically — nitrate tends to accumulate since most ponds and aquaria lack bacteria that complete denitrification to nitrogen gas, so nitrate is typically controlled through water changes or uptake by plants and algae[8][5].

References

  1. ^ UF/IFAS EDIS FA031: Ammonia in Aquatic Systems. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA031. Retrieved November 2026.
  2. ^ Understanding and Tips for Improving the Nitrogen Cycle in your Koi Pond. Kodama Koi Farm (koi dealer). kodamakoifarm.com/tips-nitrogen-cycle-koi-pond/. Retrieved November 2026.
  3. ^ Biofilter (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Biofilter. Retrieved November 2026.
  4. ^ SRAC Publication No. 4502: How to Start a Biofilter. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4502%20How%20to%20Start%20a%20B…. Retrieved November 2026.
  5. ^ Aquarium filter (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Aquarium_filter. Retrieved November 2026.
  6. ^ SRAC Publication No. 451: Recirculating Aquaculture Tank Production Systems: An Overview. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/451%20Recirculating%20Aquacultu…. Retrieved November 2026.
  7. ^ Nitrogen cycle (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Nitrogen_cycle. Retrieved November 2026.
  8. ^ Fishkeeping (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Fishkeeping. Retrieved November 2026.
  9. ^ Koi pond (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Koi_pond. Retrieved November 2026.
  10. ^ Old Pond Syndrome: Why Your Koi May Appear to Die Randomly & Potential Solutions. Kodama Koi Farm (koi dealer). kodamakoifarm.com/old-pond-syndrome-koi-death-random/. Retrieved November 2026.