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Unionized Ammonia Risk Calculator.

Calculate unionized ammonia (NH₃) from total ammonia nitrogen (TAN), pH, temperature, and salinity. 4-tier risk assessment with corrective action recommendations for aquatic facilities.

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Water Parameters

Total Ammonia Nitrogen — TAN (mg/L)

pH: 7.2

6.07.08.09.010.0

Temperature: 28.5 °C

20°C25°C28.5°C33°C

Include salinity correction

When to use

Determining the toxic fraction of ammonia from a TAN test kit reading
Assessing risk level after routine water quality testing
Evaluating the impact of pH or temperature changes on ammonia toxicity
Generating corrective action recommendations for elevated ammonia

Do not use for

As a substitute for direct water quality testing — always measure TAN with a validated kit
For marine fish species without adjusting thresholds — species-specific LC50 values vary
To replace professional veterinary consultation for acute fish mortality events

TAN vs NH₃ — know what your test kit measures

Most commercial test kits (API, Hach, Nessler) report Total Ammonia Nitrogen (TAN), not unionized ammonia. You must use pH and temperature to calculate the toxic NH₃ fraction. Reporting TAN alone without calculating NH₃ can be misleading.

Temperature and pH are multiplicative risk factors

A simultaneous increase in both pH and temperature dramatically increases the NH₃ fraction. For example, going from pH 7.0/25°C to pH 8.0/30°C increases the toxic fraction by roughly 10-fold. Always measure all three parameters together.

Morning vs afternoon measurements differ

In systems with algae or plants, pH rises during the day due to photosynthetic CO₂ uptake. An afternoon reading at pH 8.5 may show dangerous NH₃ even though the morning reading at pH 7.5 was safe. For worst-case assessment, measure in the afternoon.

Biofilter cycling takes 4–6 weeks

New systems without established nitrifying bacteria will accumulate ammonia rapidly. Do not stock at full density until the biofilter is cycled. Monitor TAN daily during the cycling period.

Method

Unionized fraction calculated using Emerson et al. (1975) pKa formula: pKa = 0.09018 + 2729.92 / (T + 273.15). Salinity correction per Khoo et al. (1977). NH₃ fraction = 1 / (1 + 10^(pKa − pH)). Risk thresholds: safe (<0.02 mg/L), caution (0.02–0.05), danger (0.05–0.2), lethal (≥0.2).

Published

Published 2026-04-06. Calculations support planning and documentation; verify procurement decisions against manufacturer specifications or institutional SOPs.

How to cite

How to Cite

ConductScience Unionized Ammonia Risk Calculator. ConductScience, Inc. 2026. Available at: https://conductscience.com/tools/unionized-ammonia-calculator

Emerson K et al. Aqueous ammonia equilibrium calculations: effect of pH and temperature. J Fish Res Board Can. 1975;32:2379–2383.

Khoo KH et al. Thermodynamics of the dissociation of ammonium ion in seawater from 5 to 40°C. J Solution Chem. 1977;6:281–290.

Ammonia Equilibrium Chemistry

In aqueous solution, ammonia exists in a pH- and temperature-dependent equilibrium:

NH₃ + H₂O ⇌ NH₄⁺ + OH⁻

The fraction of total ammonia that is unionized (NH₃) is determined by the dissociation constant (pKa) and the water pH:

Fraction NH₃ = 1 / (1 + 10^(pKa − pH))

The pKa of ammonia decreases with increasing temperature (Emerson et al., 1975):

pKa = 0.09018 + 2729.92 / (T + 273.15)

where T is temperature in °C. This means that at higher temperatures, a greater fraction of TAN exists as toxic NH₃. Salinity further reduces pKa (Khoo et al., 1977), increasing the NH₃ fraction in brackish or marine systems.

Zebrafish Ammonia Toxicity

Zebrafish are relatively sensitive to ammonia compared to many other freshwater species. Key toxicity thresholds:

< 0.02 mg/L NH₃: Safe for long-term culture
0.02–0.05 mg/L NH₃: Sublethal stress; reduced growth, altered behavior, and immune suppression
0.05–0.2 mg/L NH₃: Dangerous; gill hyperplasia, increased mucus production, fin erosion
> 0.2 mg/L NH₃: Potentially lethal; acute gill necrosis, neurological damage

Larvae and embryos are more sensitive than adults. Chronic low-level exposure can confound experimental results by inducing stress-related gene expression changes, making ammonia monitoring essential for reproducible research.

Frequently asked

What is unionized ammonia and why is it dangerous to fish?

Total ammonia in water exists in two forms: ionized ammonium (NH₄⁺) and unionized ammonia (NH₃). Unionized ammonia is the toxic fraction because it is uncharged and can freely cross gill membranes, damaging tissue and disrupting ion regulation. Even low concentrations (>0.02 mg/L) can cause chronic stress, reduced growth, and increased disease susceptibility in zebrafish.

Why does pH affect ammonia toxicity?

The equilibrium between NH₃ and NH₄⁺ is pH-dependent. At higher pH, a greater fraction of total ammonia exists as toxic NH₃. For example, at pH 7.0 only ~0.5% of TAN is NH₃, but at pH 8.5 this rises to ~10–15%. This is why pH control is critical in recirculating aquaculture systems — a small pH increase can dramatically increase the toxic fraction.

What is Total Ammonia Nitrogen (TAN)?

TAN (Total Ammonia Nitrogen) is the sum of unionized ammonia (NH₃-N) and ionized ammonium (NH₄⁺-N) expressed as nitrogen. Most commercial test kits report TAN directly. To determine the actual toxic NH₃ concentration, you need to know the water pH and temperature to calculate the fraction that is unionized.

What are safe ammonia levels for zebrafish?

For zebrafish facilities, unionized ammonia (NH₃) should be kept below 0.02 mg/L for chronic exposure. Levels above 0.05 mg/L require corrective action, and levels above 0.2 mg/L are considered dangerous. The lethal concentration varies by exposure duration but is generally above 0.2 mg/L NH₃. Total ammonia nitrogen (TAN) should ideally remain below 0.1 mg/L in a well-maintained system.

How can I lower ammonia levels in my aquatic facility?

Immediate actions include water changes (25–75% depending on severity) and reducing or stopping feeding. Long-term management requires a properly cycled biofilter with adequate nitrifying bacteria (Nitrosomonas and Nitrobacter), appropriate stocking density, removal of dead organisms and uneaten food, and regular filter maintenance. Lowering pH (within safe range) also shifts the equilibrium toward less-toxic NH₄⁺.

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