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Ammonia

By the koi.wiki editorial team · Updated

Key facts: Ammonia is a nitrogenous waste product excreted by koi through the gills, existing in water as a toxic un-ionized form (NH3) and a far less toxic ionized form (NH4+), with the balance between them governed mainly by pH and temperature.[1][2]
Ammonia accumulates in koi ponds as a natural byproduct of fish metabolism and is continuously excreted through the gills, so it must be regularly measured and removed before it damages or kills fish.[1] It is colorless and odorless even at toxic levels, meaning testing is the only way to detect it.[1] Total ammonia nitrogen (TAN) in water exists as two chemical forms, un-ionized ammonia (NH3) and ionized ammonium (NH4+), and the un-ionized form is roughly 100 times more toxic to fish than the ionized form.[1]The proportion of TAN present as toxic un-ionized ammonia rises as pH and temperature increase, and, to a lesser extent, falls as salinity increases.[1] This relationship is central to understanding and managing ammonia risk in any koi pond, and is closely tied to pH, temperature, and the nitrogen cycle.[2][1]

Forms of ammonia in water

Total ammonia nitrogen (TAN) is the sum of un-ionized ammonia (NH3) and ionized ammonium (NH4+).[3][6] Un-ionized ammonia is the toxic form and predominates when pH is high, while the ionized ammonium ion is relatively non-toxic and predominates when pH is low; generally less than 10% of ammonia is in the toxic form when pH is below 8.0, but this proportion increases dramatically as pH rises further.[2]According to Wikipedia, ammonia is a polar, trigonal pyramidal molecule that is highly miscible with water and acts as a weak base; a 1.0 M aqueous solution has a pH of about 11.6.[4] Shifts in environmental pH change the proportion of ammonium present versus un-ionized ammonia in any aqueous system.[4]

Effect of pH and temperature on toxicity

For every one-unit increase in pH, the amount of toxic un-ionized ammonia increases by roughly ten times.[3] Higher water temperature also increases the un-ionized fraction, though pH has the larger effect.[2][7] Reference tables based on work by Emerson et al.High total alkalinity water also makes ammonia more toxic because alkalinity raises pH, and liming ponds to raise pH can therefore worsen an existing ammonia problem.[8][2] Hardness itself is not typically associated with ammonia toxicity, but alkalinity is.[8]

Toxic thresholds

According to the University of Florida IFAS Extension, damage to fish tissue can begin once un-ionized ammonia (UIA) exceeds 0.05 mg/L, with harm increasing as concentration rises further; at 2.0 mg/L, sensitive fish typically die.[1] The Southern Regional Aquaculture Center states that short-term levels capable of killing fish over a few days start at about 0.6 mg/L UIA, while chronic exposure to levels as low as 0.06 mg/L can cause gill and kidney damage, reduced growth, and possible brain and oxygen-carrying impairment.[3]Wikipedia notes that around 0.05 mg/L of un-ionized ammonia is harmful to fish, causing poor growth and feed conversion, reduced fecundity and fertility, and increased stress and susceptibility to disease, while levels above 2.0 mg/L cause gill and tissue damage, lethargy, convulsions, coma, and death; lethal concentrations across fish species generally range from 0.2 to 2.0 mg/L.[4] For recirculating systems, un-ionized ammonia-nitrogen concentrations should generally not exceed 0.05 mg/L, according to the Southern Regional Aquaculture Center.[5] Even sub-lethal ammonia exposure makes fish more susceptible to infectious disease.[1]

Testing for ammonia

Most commercial test kits measure TAN in mg/L (equivalent to parts per million), and the un-ionized fraction must then be calculated from TAN together with measured pH and temperature.[1] A good ammonia test kit therefore includes a TAN test, a pH test, and a thermometer.[1] Two common laboratory methods are the Nessler's method and the ammonia salicylate method; the Nessler's method can give falsely elevated readings if formalin or certain water conditioners have been used recently, and is unsuitable for seawater, while its reagent contains mercury requiring hazardous-waste disposal in many US states.[1]Diagnostic laboratories evaluating sick or dying koi typically test both total and unionized ammonia alongside nitrite, pH, hardness, alkalinity, and chloride as part of a general water chemistry screen.[9] Investigators of fish die-offs are likewise advised to evaluate temperature, dissolved oxygen, total ammonia, pH, and nitrite immediately, since water quality can change considerably within hours of a mortality event.[10]

Role of the nitrogen cycle

Ammonia is removed from water by nitrification, a biological process in which ammonia-oxidizing bacteria such as Nitrosospira and Nitrosomonas convert ammonia to nitrite, and nitrite-oxidizing bacteria such as Nitrospira and Nitrobacter convert nitrite to the much less toxic nitrate.[1] This aerobic process also produces carbon dioxide and free hydrogen ions, which can lower pH unless sufficient alkalinity buffers the water.[1] In tanks or ponds, the bacteria colonize a biofilter or substrate surfaces, and a new biofilter typically requires six to eight weeks to build up enough bacteria to effectively reduce ammonia and nitrite.[1] Ammonia-oxidizing bacteria possess enzymes called ammonia monooxygenases that catalyze the first step of this conversion, and this enzymatic activity is itself sensitive to pH, temperature, and oxygen availability.[11] For background on this process, see the nitrogen cycle and nitrite.[1]

Managing ammonia problems

When ammonia is detected, fish should not be fed, or feeding should be reduced, until the problem is corrected.[1] In small systems a water change may help in the short term, and ammonia-binding products can alleviate toxicity temporarily, but a properly designed and sized biofilter is the long-term solution.[1] In large ponds, a 0–20–0 phosphate fertilizer applied at about 40 pounds per acre may stimulate phytoplankton growth that removes ammonia over several days, though it will not help in an acute crisis and will not work if phosphorus is not a limiting factor or if a heavy algae bloom is already present.[1] Regular testing is important because once ammonia problems are severe or fish begin dying, it becomes difficult to correct the situation without further losses, and fish may sicken weeks after an ammonia event has been resolved.[1]Aeration is sometimes believed to help by accelerating the diffusion of ammonia gas from water to air, but research summarized by the Southern Regional Aquaculture Center found aeration largely ineffective at reducing ammonia concentration in ponds, and intensive aeration can even increase ammonia by resuspending sediments.[2] Adding lime to ponds can also worsen ammonia problems because it raises pH, shifting ammonia toward its more toxic un-ionized form.[2]

Ammonia during transport and handling

Ammonia is a particular concern when koi are bagged or hauled, since metabolic waste accumulates in a closed volume of water.[12] As pH and temperature decrease, the proportion of ammonia in the toxic un-ionized form also decreases, so shippers commonly keep hauling water pH around 7 to 7.5 and use well-buffered water to limit swings.[13] Carbon dioxide buildup during transport lowers pH by forming carbonic acid, which shifts the ammonia balance toward the less toxic ionized form, though this effect reverses as CO2 off-gasses after fish are released.[7] Common shipping practices to limit ammonia include reducing fish biomass per bag, withholding food for 24 hours before shipment, shortening transport time, keeping water cool, and using chemical ammonia-binding additives such as sodium hydroxymethanesulfonate.[12] For related guidance, see pH and shipping, bagging and acclimation.[7][13]

Common questions

Why is un-ionized ammonia more dangerous than total ammonia readings suggest?
Standard test kits measure total ammonia nitrogen (TAN), but only the un-ionized ammonia (NH3) fraction is highly toxic, being roughly 100 times more toxic than the ionized ammonium (NH4+) form; the un-ionized fraction must be calculated from TAN, pH, and temperature.[1]
At what ammonia level do koi start to suffer harm?
Damage to fish tissue can begin once un-ionized ammonia exceeds about 0.05 mg/L, with sensitive fish typically dying around 2.0 mg/L, according to University of Florida IFAS Extension and the Southern Regional Aquaculture Center.[1][3]
Does raising pond pH make ammonia worse?
Yes; higher pH shifts the ammonia balance toward the toxic un-ionized form, so practices like adding lime that raise pH can worsen an existing ammonia problem.[2][8]
How long does it take a new biofilter to control ammonia?
A newly established biofilter typically requires about six to eight weeks to build up enough nitrifying bacteria to effectively reduce ammonia and nitrite, according to UF/IFAS Extension, while a related source cites at least one month for activation.[1][14]
Is aeration an effective way to reduce ammonia in a pond?
Research summarized by the Southern Regional Aquaculture Center found aeration generally ineffective at lowering pond ammonia concentrations, and vigorous aeration can even raise ammonia by stirring up sediment.[2]

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. ^ SRAC Publication No. 4603: Managing Ammonia in Fish Ponds. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4603%20Managing%20Ammonia%20in%…. Retrieved November 2026.
  3. ^ SRAC Publication No. 463: Ammonia in Fish Ponds. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/463%20Ammonia%20in%20Fish%20Pon…. Retrieved November 2026.
  4. ^ Ammonia (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Ammonia. Retrieved November 2026.
  5. ^ 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.
  6. ^ 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.
  7. ^ UF/IFAS EDIS FA119: On-Farm Transport of Ornamental Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA119. Retrieved November 2026.
  8. ^ SRAC Publication No. 464: Interactions of pH, Carbon Dioxide, Alkalinity and Hardness in Fish Ponds. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/464%20Interactions%20of%20pH%2C…. 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. ^ SRAC Publication No. 472: Investigating a Fish Die-Off and Submitting a Sample. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/472%20Investigating%20a%20Fish%…. Retrieved November 2026.
  11. ^ Nitrification (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Nitrification. Retrieved November 2026.
  12. ^ SRAC Publication No. 3903: Shipping Fish in Boxes. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/3903%20Shipping%20Fish%20in%20B…. Retrieved November 2026.
  13. ^ SRAC Publication No. 392: Transportation of Warmwater Fish: Procedures and Loading Rates. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/392%20Transportation%20of%20War…. Retrieved November 2026.
  14. ^ 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.