Water-quality reference ranges
Key facts: This page compiles reference ranges for key pond water-quality parameters—ammonia, nitrite, pH, alkalinity (KH), dissolved oxygen and temperature—as reported by university and government aquaculture extension sources, which primarily describe warmwater fish and catfish/aquaculture ponds rather than koi specifically.
[1][2][3][4][5]
Water-quality reference ranges used by koi keepers are often adapted from aquaculture extension literature covering warmwater pond fish generally, since dedicated koi-specific toxicological studies are limited in the cited sources. The Southern Regional Aquaculture Center (SRAC) and University of Florida IFAS Extension (EDIS) publish guidance for parameters including total ammonia nitrogen, nitrite, pH, total alkalinity, dissolved oxygen and temperature effects, largely in the context of catfish and other pond aquaculture.[1][2][3][4][5]These sources note that several parameters—particularly dissolved oxygen, pH and carbon dioxide—fluctuate over a 24-hour cycle due to photosynthesis and respiration, and that on-site, same-day measurement is recommended because laboratory shipment can change results for some analytes.[1][4][1][2][3][4][5]
Reference ranges table
| Parameter | Reported range / threshold | Source | Notes |
|---|---|---|---|
| Total ammonia nitrogen (TAN) | Maximum safe levels vary by pH and temperature, e.g. at pH 7.0 the acute (1-hour) limit is 36.0 mg N/L at 64°F (18°C) and 3.2 mg N/L at 86°F (30°C) for warmwater fish; chronic limits are much lower. | [1] | Adapted from U.S. EPA (1999) criteria for warmwater fish, not koi-specific. |
| Un-ionized ammonia (UIA) | Tissue damage can occur above 0.05 mg/L; at 2.0 mg/L sensitive fish typically die. Any detectable ammonia is considered a problem requiring correction. | [2] | UIA is calculated from TAN, pH and temperature using a multiplication-factor table. |
| Nitrite | No single numeric safe limit is given; toxicity (brown blood disease) is managed via a chloride-to-nitrite ratio of 10:1 to prevent nitrite uptake by fish. | [5] | Sensitivity varies by species; catfish and tilapia are described as fairly sensitive, trout and other coolwater fish as sensitive to very small amounts. |
| pH | Most fish species do well between 6.5 and 9.0 according to SRAC 4606; SRAC 464 states a desirable range close to fish blood pH, about 7.0 to 8.0, with stress/death below 5 or above 10. | [1][4] | Sources differ slightly on the ideal band; both describe general pond fish, not koi specifically. |
| Total alkalinity (KH) | Desirable range 50–150 mg/L as CaCO₃; acceptable range more than 20 mg/L and less than 400 mg/L for ponds. SRAC 464 separately states a desirable total alkalinity of 75–200 mg/L CaCO₃ for fish culture. | [1][4] | UF/IFAS notes nitrifying bacteria function poorly below 20 mg/L alkalinity ("old tank syndrome"). |
| Alkalinity (biofilter function) | Below 20 mg/L, common nitrifying bacteria do not function effectively, and pH can drop rapidly, killing fish and bacteria. | [2] | Applies particularly to recirculating systems. |
| Dissolved oxygen (DO) | 5 mg/L or more recommended for optimum fish health; most species are distressed at 2–4 mg/L; mortality usually occurs below 2 mg/L; emergency aeration recommended below 4 mg/L. | [3] | DO follows a diurnal cycle, rising during the day and falling at night. |
| Dissolved oxygen (nitrification) | Should be kept above 5 mg/L to support nitrifying bacteria function; low DO limits ammonia/nitrite conversion. | [2] | Overlaps with general fish-health DO guidance. |
| Temperature | At 90°F (32°C) water holds about 7.4 mg/L DO at saturation versus 11.9 mg/L at 45°F (7°C); warmer water increases fish metabolic oxygen demand while reducing oxygen-holding capacity. | [3] | Figures given in the source are specific temperature-saturation data points, not a koi-specific range. |
How parameters interact
Alkalinity, pH and dissolved carbon dioxide are described as interrelated: during daylight, algae and plants consume carbon dioxide for photosynthesis, raising pH; at night, carbon dioxide accumulates from respiration and pH falls. Water with moderate to high alkalinity is described as better buffered against these daily swings, remaining neutral to slightly basic (7.0–8.3) without wide fluctuation.[4]Ammonia toxicity is also pH- and temperature-dependent: as pH or temperature rises, a greater proportion of total ammonia nitrogen exists as the more toxic un-ionized form, so the same TAN reading can represent very different risk levels depending on these two factors.[1][2]Dissolved oxygen and nitrification are linked as well: both ammonia-oxidizing and nitrite-oxidizing bacteria require oxygen and alkalinity to function, so low DO or depleted alkalinity can cause ammonia and nitrite to accumulate even in an otherwise established system, a phenomenon described as "old tank syndrome."[2]Measurement notes and limitations
Several cited sources caution that routine laboratory water analysis does not reliably capture some of the most important parameters. Dissolved oxygen in particular should be measured on-site, since it may change dramatically during shipment of a sample to a laboratory, and it is described as the leading cause of fish kills when low.[1]pH is also best measured promptly, ideally within 30 minutes of sampling, because biological activity in a water sample can alter it before laboratory analysis.[4]The reference values above are drawn from aquaculture extension literature focused mainly on catfish and general warmwater pond fish culture; the sources do not provide koi-specific toxicological thresholds, so figures should be treated as general guidance rather than species-specific standards.[1][5]Species differences in sensitivity
Sensitivity to nitrite is reported to vary by species: catfish and tilapia are described as fairly sensitive, while trout and other coolwater fish are sensitive to extremely small amounts of nitrite; goldfish and fathead minnows are described as falling between catfish and bass in susceptibility to brown blood disease.[5]Because koi are not specifically addressed in these publications, keepers consulting this table should treat the figures as a general aquaculture baseline rather than a koi-specific standard, and should consult the dedicated parameter pages for ammonia, nitrite, pH, KH (alkalinity), dissolved oxygen and temperature for further context.[1][2][3]Related management practices
When ammonia is detected, UF/IFAS recommends reducing or stopping feeding for one to three days and, in small systems, performing a 25% to 50% water change, a practice also discussed on the page about water changes.[2]Because biofilter performance depends on oxygen and alkalinity, keepers may also want to review pages on pond filtration, biological filter media and aeration and air pumps alongside this reference table.[2][3]Sudden pH drops linked to depleted alkalinity are described as a risk in recirculating systems, which relates to practices covered on the KH (alkalinity) and baking soda page.[2]Common questions
What ammonia level is dangerous for koi?
UF/IFAS states that un-ionized ammonia above 0.05 mg/L can damage fish tissue, with sensitive fish typically dying at 2.0 mg/L, and notes that any detectable ammonia indicates a problem requiring corrective action.[2]
What is a safe dissolved oxygen level for a koi pond?
UF/IFAS recommends 5 mg/L or more for optimum fish health, with distress typically occurring at 2–4 mg/L and mortality below 2 mg/L; emergency aeration is recommended if DO falls below 4 mg/L.[3]
What pH range is recommended for fish ponds?
Why does alkalinity (KH) matter for water quality?
Does nitrite have a universal safe limit?
The cited source does not give a single numeric safe nitrite limit; instead it describes managing nitrite toxicity (brown blood disease) by maintaining a chloride-to-nitrite ratio of 10:1, and notes that sensitivity varies considerably between fish species.[5]
Related koi topics
References
- ^ 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.
- ^ UF/IFAS EDIS FA031: Ammonia in Aquatic Systems. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA031. Retrieved November 2026.
- ^ UF/IFAS EDIS FA002: Dissolved Oxygen for Fish Production. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA002. Retrieved November 2026.
- ^ 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.
- ^ 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.