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Nitrite

By the koi.wiki editorial team · Updated

Key facts: Nitrite (NO2−) is an intermediate compound in the nitrogen cycle, produced when nitrifying bacteria oxidize ammonia; it is toxic to fish, causing “brown blood disease” by binding hemoglobin into methemoglobin, which cannot carry oxygen[1][2].
In a koi pond, nitrite forms when ammonia-oxidizing bacteria such as Nitrosomonas convert fish-excreted ammonia into nitrite, which is then normally oxidized further into much less toxic nitrate by nitrite-oxidizing bacteria such as Nitrobacter[2][6]. When this second step lags—because a biofilter is immature, overloaded, or disrupted—nitrite can accumulate to dangerous levels[1][7].Nitrite is toxic because it crosses the gills into the bloodstream and reacts with hemoglobin to form methemoglobin, a form of blood pigment that cannot transport oxygen[1][6]. Affected fish can suffocate even when dissolved oxygen in the water is adequate, and may be seen gasping at the surface[1]. This condition is commonly called brown blood disease because the blood itself turns a chocolate-brown color[1][7].

How nitrite forms in a pond

Nitrite is produced during nitrification, the second stage of the nitrogen cycle, after ammonia-oxidizing bacteria convert fish waste and decomposing feed into ammonia and then nitrite[2]. Under normal conditions nitrite-oxidizing bacteria quickly convert it to non-toxic nitrate, but nitrite spikes are more likely in closed, intensive systems with insufficient, inefficient, or malfunctioning filtration[1]. High nitrite is also more common in fall and spring, when fluctuating temperatures and reduced plankton or bacterial activity disrupt the nitrogen cycle[1]. A newly set up biofilter typically needs six to eight weeks to build sufficient bacteria to control both ammonia and nitrite[2].Nitrification is also acid-producing: as ammonia is converted to nitrate, hydrogen ions are released, consuming alkalinity and lowering pH unless adequate buffering is present[6][2]. See the nitrogen cycle and KH (alkalinity) pages for related background, and ammonia for the preceding stage.

Brown blood disease and methemoglobin

When nitrite enters the bloodstream through the gills, it combines with hemoglobin to form methemoglobin, which is incapable of carrying oxygen; this turns the blood a chocolate-brown color, giving the condition its name[1]. Fish with brown blood disease may gasp at the surface even when water oxygen levels are relatively high, because the blood itself cannot deliver oxygen to tissues[1]. In recirculating systems, symptoms linked to high ammonia or nitrite include erratic swimming, lying at the surface, crowding around aerators, gulping at the surface, and reduced or stopped feeding[7]. Fish that survive a brown blood episode may become more susceptible to bacterial infections such as Aeromonas and Columnaris, anemia, and other stress-related diseases, sometimes occurring one to three weeks afterward[1]. See Aeromonas and ulcers and Columnaris for these secondary conditions, and gill problems for related gill damage.

Toxicity thresholds and species sensitivity

Sensitivity to nitrite varies greatly by species. In general aquaculture literature, catfish are described as fairly sensitive, with nitrite-nitrogen as low as 0.5 ppm considered stressful, while tilapia tolerate concentrations under 5 ppm with little stress[7]. Trout and other coolwater fish are described as sensitive to extremely small amounts of nitrite, whereas largemouth and smallmouth bass, bluegill, and green sunfish are resistant[1]. One source notes that scaled species are generally more tolerant of high nitrite than species like catfish[7]. For general fish culture, a desirable range for nitrite-nitrogen is less than 0.1 mg/L, with typical pond concentrations of 0.005 to 0.5 mg/L nitrite-nitrogen[4]. A separate source states nitrite is toxic to fish at levels as low as 0.10 mg/L[2]. None of the cited sources give a nitrite tolerance figure specific to koi or common carp; keepers testing for nitrite in a koi pond should rely on a test kit and interpret results cautiously given this species-specific variability.

Chloride and salt as treatment

Chloride ions compete with nitrite for absorption through the gills, and maintaining an adequate chloride-to-nitrite ratio is the standard method for preventing and treating brown blood disease[1][6]. A commonly cited minimum ratio is 10 parts chloride to 1 part nitrite, though a higher 20:1 ratio is sometimes listed as more fully protective, and species with greater nitrite sensitivity may need an even higher ratio[1][4][6]. One source frames the treatment as adding 5 to 6 ppm chloride for each 1 ppm nitrite present, recommending a water exchange and reduced feeding alongside the salt addition when nitrite exceeds 0.5 ppm[7]. Sodium chloride (common salt) is the chloride source typically used; calcium chloride can also be used but is usually more expensive[1]. It takes about 24 hours after salt is applied for the brown blood condition to be alleviated[1]. Some facilities maintain a standing chloride “insurance” level, commonly around 100 mg/L, so that chloride is already present as a buffer if nitrite spikes suddenly[1][4]. Similarly, freshwater aquaculturists sometimes maintain salt at 1–3 g/L (0.1–0.3%) in a system partly to help prevent brown blood disease caused by excess nitrite, in addition to other benefits such as reduced parasite loads[8].The amount of salt needed can be calculated from pond or tank volume and the measured chloride and nitrite concentrations, using formulas published by aquaculture extension sources; commercial water test kits are used to obtain these readings[1]. Chloride levels can fluctuate after heavy rain or well-water flushing, which dilutes chloride and reduces the chloride-to-nitrite ratio, so re-checking after such events is advised[1].

Methylene blue as an emergency treatment

Aside from salt, methylene blue is described as working against nitrite poisoning in fish because it treats the resulting methemoglobinemia, administered as a medicated bath[5]. In human medicine, methylene blue is used to chemically reduce the ferric iron in methemoglobin back to the ferrous form found in normal hemoglobin, restoring oxygen-carrying capacity; it is specifically indicated when methemoglobin levels are high or symptomatic[5]. However, the same source notes that methylene blue is not without side effects for fish, and that non-professional claims about its use for ammonia poisoning lack strong supporting literature, so its use for nitrite-related problems in koi should be understood as a chemical antidote for methemoglobinemia specifically, separate from fixing the underlying water-quality imbalance[5].

Monitoring and management

Because nitrite problems can develop quickly, extension guidance recommends checking pond water for nitrite two to three times a week during fall and spring, and at least weekly the rest of the year, with daily checks during a known high-nitrite incident[1]. Total ammonia nitrogen (TAN) can also be monitored as a leading indicator, since every 1 ppm of TAN can convert to roughly 3 ppm of nitrite within a relatively short period[1]. In recirculating systems, ammonia and nitrite should both be tested more frequently after any disease treatment, since chemicals such as antibiotics can harm the nitrifying bacteria in a biofilter[2]. General protocols for intensive systems call for recording ammonia, nitrite, pH, alkalinity, dissolved oxygen, temperature, hardness, and salinity, aiming to keep ammonia and nitrite at undetectable levels[8]. Reducing feeding rates is also cited as a direct way to lower the nitrogen load entering the system when nitrite is elevated[1][7]. For broader context on routine testing, see test kits and monitors and water-quality reference ranges; related waste-product pages include ammonia and nitrate.

Chemistry of the nitrite ion

Chemically, nitrite is the anion NO2−, the conjugate base of the weak acid nitrous acid[3]. It is a pervasive intermediate in the nitrogen cycle in nature and is also produced and used industrially, for example as sodium nitrite in chemical and pharmaceutical applications[3]. In fish diagnostic work, nitrite is one of the standard water chemistry parameters tested alongside total and unionized ammonia, pH, hardness, alkalinity, and chloride when a water sample or fish is submitted for evaluation[9]. Chloride itself is a different ion from the chlorine gas used as a disinfectant, and forms common salt when paired with sodium[4][10].

Common questions

What is brown blood disease in koi?
It is the common name for nitrite toxicity, in which nitrite entering the bloodstream through the gills converts hemoglobin into methemoglobin, turning the blood brown and preventing it from carrying oxygen, so affected fish may gasp even in well-oxygenated water[1].
How much salt is needed to counter high nitrite?
Extension sources recommend maintaining a chloride-to-nitrite ratio of at least 10:1, with one source citing 20:1 as more fully protective, and another suggesting adding about 5–6 ppm chloride for each 1 ppm of nitrite present, calculated from pond or tank volume and measured concentrations[1][4][7].
How long does it take for salt to relieve brown blood disease?
According to one extension publication, it takes about 24 hours after salt is applied to a pond for the brown blood condition to be alleviated[1].
Can methylene blue treat nitrite poisoning in koi?
According to a secondary source, methylene blue can work against nitrite poisoning because it treats the resulting methemoglobinemia, though it is not without side effects to fish[5].
How often should nitrite be tested in a koi pond?
Extension guidance for fish ponds recommends checking nitrite two to three times per week in fall and spring and at least weekly otherwise, with daily testing during a known high-nitrite event[1].

References

  1. ^ SRAC Publication No. 462: Nitrite in Fish Ponds. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/462%20Nitrite%20in%20Fish%20Pon…. Retrieved November 2026.
  2. ^ UF/IFAS EDIS FA031: Ammonia in Aquatic Systems. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA031. Retrieved November 2026.
  3. ^ Nitrite (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Nitrite. Retrieved November 2026.
  4. ^ SRAC Publication No. 4606: Interpretation of Water Analysis Reports for Fish Culture. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4606%20Interpretation%20of%20Wa…. Retrieved November 2026.
  5. ^ Methylene blue (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Methylene_blue. 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. ^ SRAC Publication No. 452: Recirculating Aquaculture Tank Production Systems: Management. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/452%20Recirculating%20Aquacultu…. Retrieved November 2026.
  8. ^ UF/IFAS EDIS FA101: Fish Health Management Considerations in Recirculating Aquaculture Systems—Part 3. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA101. Retrieved November 2026.
  9. ^ UF/IFAS EDIS FA055: Submission of Fish for Diagnostic Evaluation. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA055. Retrieved November 2026.
  10. ^ Salt (chemistry) (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Salt_(chemistry). Retrieved November 2026.