How do you calculate free ammonia in a koi pond?

Free ammonia (NH₃) is calculated from total ammonia nitrogen (TAN) using the temperature-dependent acid dissociation constant (pKa). The formula uses pKa = 0.09018 + 2729.92/(T+273.15) where T is temperature in Celsius, then NH₃ = TAN / (1 + 10^(pKa - pH)). Higher pH and higher temperature increase the toxic NH₃ fraction.

What free ammonia level is dangerous for koi?

Free ammonia (NH₃) above 0.02 mg/L causes chronic stress in koi. Above 0.05 mg/L, active physiological damage occurs including gill epithelial damage and reduced feeding. Above 0.2 mg/L, mortality is likely without immediate intervention.

Ammonia Toxicity Calculator

Koi-Specific Thresholds Worker-Powered

This calculator determines the toxic free ammonia (NH₃) concentration in your koi pond from your total ammonia nitrogen (TAN) test reading, pH, and water temperature. It compares results against both EPA aquatic life criteria and koi-specific sublethal stress thresholds, providing a risk assessment and recommended actions.

Enter Your Water Parameters

Total Ammonia Nitrogen (TAN) mg/L

Water Temperature

Temperature Unit

How This Calculator Works

The calculator sends your inputs to a Cloudflare Worker that performs the ammonia equilibrium calculation using the temperature-dependent pKa derived from the Emerson et al. (1975) equation. It then compares your total ammonia nitrogen against both the EPA 2013 freshwater ammonia criteria (acute CMC and chronic CCC) and koi-specific sublethal thresholds derived from published fish toxicology research.

The calculator does not expose the underlying formulas in your browser — all computation happens server-side. This ensures calculation accuracy and prevents formula manipulation.

Understanding Your Results

Free Ammonia (NH₃)

This is the toxic fraction of your total ammonia reading. It is the number that determines whether your fish are in danger. A TAN reading of 2.0 mg/L might represent 0.01 mg/L free ammonia (safe at low pH) or 0.30 mg/L free ammonia (lethal at high pH). Context is everything.

EPA Thresholds

The EPA Criterion Maximum Concentration (CMC) is the acute threshold — exceeding this level risks rapid mortality. The Criterion Continuous Concentration (CCC) is the chronic threshold — sustained exposure above this level causes long-term harm. Both are calculated dynamically based on your pH and temperature.

Risk Levels

The calculator assigns one of four risk levels based on both the free ammonia concentration and EPA threshold comparisons: Low Risk (safe), Moderate Risk (sublethal stress possible), High Risk (acute stress and mortality likely), and Critical Risk (fish kill imminent).

Related Resources

Ammonia & Nitrite in Koi Ponds — Comprehensive guide to ammonia toxicity, thresholds, and treatment protocols.
The Nitrogen Cycle — Understanding the biological process that converts ammonia to less toxic forms.
pH, KH & GH — Why pH management is inseparable from ammonia management.
Salt Therapy — Using salt to protect against nitrite toxicity during nitrogen cycle disruptions.

Emerson, K., Russo, R.C., Lund, R.E., & Thurston, R.V. (1975). Aqueous ammonia equilibrium calculations: Effect of pH and temperature. Journal of the Fisheries Research Board of Canada, 32(12), 2379–2383.
USEPA (2013). Aquatic Life Ambient Water Quality Criteria for Ammonia — Freshwater (EPA 822-R-13-001). United States Environmental Protection Agency.
Thurston, R.V., Russo, R.C., & Vinogradov, G.A. (1981). Ammonia toxicity to fishes. Environmental Science & Technology, 15(7), 837–840.