Fish care calculator
Ammonia Toxicity Calculator (Free NH3) | FishZone
Enter your test kit reading (TAN), pH, and temperature — get an instant SAFE or LETHAL verdict for Free Ammonia (NH3). Built for API Master Kit users, cichlid tanks, reef setups, and fish breeders.
- Scientific Formula
- Reviewed by Experts
- Updated May 2026
- Instant Results
Type
Risk Assessment
Inputs
4
Difficulty
Beginner
Calculation Time
Instant
Quick Answer
Liquid test kits measure Total Ammonia Nitrogen (TAN), a mix of harmless ammonium (NH4+) and toxic free ammonia (NH3). The NH3 percentage depends on pH and temperature. At 25°C/77°F, a TAN reading of 2.0 ppm gives <0.001 ppm NH3 at pH 6.0 (safe) but ~0.1 ppm NH3 at pH 8.0 (lethal above 0.05 ppm). Always factor in both pH and temp. The underlying formula is: NH3 = TAN / (10^(pKa − pH) + 1), where pKa ≈ 9.25 at 25°C. The calculator handles this automatically, adjusting pKa for your water temperature.
Enter your details
Enter the labeled values below. Results appear without leaving this page.
How to Use This Calculator (Worked Example)
This tool requires 4 key inputs:
- Total Ammonia Nitrogen (ppm): Enter your specific value (e.g., from your tank's test kit or dimensions).
- Tank pH Level: Enter your specific value (e.g., from your tank's test kit or dimensions).
- Tank Temperature: Enter your specific value (e.g., from your tank's test kit or dimensions).
- Temperature unit: Enter your specific value (e.g., from your tank's test kit or dimensions).
Example: If you input average baseline values, the calculator will immediately process the formula and return the recommended output and safety warnings above.
Method
Calculates free ammonia using the Emerson equation
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The Problem
Standard liquid test kits measure Total Ammonia Nitrogen (TAN), not toxic Free Ammonia (NH3). Users don't know if their reading means 'do nothing' or 'emergency water change'. This leads to either panic over-reactions or ignoring truly lethal levels.
When to Use This Tool
Use this calculator when: (1) Your API or liquid test kit shows any reading above 0 ppm ammonia. (2) Your tank pH is above 7.5, where NH3 risk rises sharply. (3) Water temperature exceeds 26°C/79°F during summer. (4) After adding new fish, overfeeding, or a filter crash. (5) During the first 6 weeks of a new tank cycle. (6) Before and after water changes in high-pH tanks to avoid Old Tank Syndrome.
Who is this for?
- Beginner fish keepers who panic at any positive ammonia test reading
- Intermediate hobbyists keeping sensitive fish (discus, reef, cichlids)
- Fish breeders and quarantine managers needing precise toxicity thresholds
- High-pH tank owners (African cichlids, marine) where NH3 is more lethal
- New tank cyclers (0–8 weeks) monitoring ammonia spikes during the nitrogen cycle
- Planted tank and CO2 injection owners where pH swings daily and NH3 risk fluctuates
About this calculator
The single most common cause of panic among new fish keepers is a positive reading on an ammonia test kit. However, most standard test kits (such as the API Master Test Kit) do not actually tell you if your fish are in danger. They measure Total Ammonia Nitrogen (TAN), but only one component of TAN is toxic.
How to Use This Calculator
- Measure TAN: Test your aquarium water with a liquid test kit (API, Salifert, or Red Sea). Note the result in ppm. Do not use test strips — they are not accurate enough for this calculation.
- Measure pH: Use a liquid pH test kit or calibrated digital pH meter at the same time as your ammonia test. pH that was measured yesterday is not reliable — pH shifts throughout the day in planted tanks and reef systems.
- Measure temperature: Use an aquarium thermometer. Enter the value in either °C or °F.
- Read your result: The calculator outputs Free Ammonia (NH3) in ppm and a risk verdict. Safe = below 0.02 ppm. Chronic damage zone = 0.02–0.05 ppm. Emergency = above 0.05 ppm.
NH3 vs NH4+: The Chemistry of Toxicity
When fish produce waste, it enters the water as ammonia. Depending on the chemical environment of the water, it exists in two forms that constantly shift back and forth in equilibrium:
- Ammonium (NH4+): Ionized ammonia. It is relatively harmless to fish because it cannot easily pass through their cell membranes and gill tissues.
- Free Ammonia (NH3): Unionized ammonia. This is the deadly form. It passes directly into the fish's bloodstream, causing chemical burns on the gills, organ failure, and rapid death.
The Role of pH and Temperature
The ratio of NH3 to NH4+ is dictated entirely by pH and temperature.
pH is the master variable. For every 1.0 drop in pH, the amount of toxic NH3 drops by a factor of 10. In acidic water (pH under 6.5), almost all ammonia exists as harmless NH4+. This is why Amazonian blackwater fish can survive in shipping bags for days. However, in alkaline water (pH above 8.0), a massive percentage of TAN converts into deadly NH3. This is why African Cichlids and marine fish are extremely vulnerable to ammonia spikes.
Temperature plays a secondary role; warmer water increases the percentage of toxic NH3, while colder water decreases it.
Real-World Example: Same TAN, Different Danger
Consider two tanks both reading 1.0 ppm TAN on an API test kit:
Tank A (pH 6.5, 24°C): Free NH3 ≈ 0.002 ppm — completely safe.
Tank B (pH 8.2, 28°C): Free NH3 ≈ 0.10 ppm — rapidly lethal (2x the acute toxicity threshold of 0.05 ppm).
Same test kit reading. Completely different outcome. This is why pH and temperature are not optional inputs.
Safe Levels at a Glance
NH3 below 0.02 ppm: Safe for most fish long-term.
0.02–0.05 ppm: Chronic gill damage zone — act within 24–48 hours.
Above 0.05 ppm: Acute toxicity — emergency water change needed immediately.
Above 0.2 ppm: Rapid death for most species within hours.
When Should You Panic?
Research shows that chronic damage to gill tissue begins when Free Ammonia (NH3) exceeds 0.02 ppm. Lethal toxicity occurs rapidly above 0.05 ppm. Use the calculator above to convert your API test kit reading (TAN) into Free Ammonia (NH3) to determine if emergency water changes are necessary.
Reference Table
Static Ammonia Toxicity Calculator (Free NH3) Reference
These baseline ratios and decision rules are included directly in the page so visitors can review core guidance before using the interactive calculator.
| Scenario | Baseline Rule | Safety Note |
|---|---|---|
| Inputs used by this tool | Total Ammonia Nitrogen (ppm), Tank pH Level, Tank Temperature, Temperature unit | The interactive calculator refines the result from these inputs. |
| Outputs generated | Calc error, Safety status, Safety advice, Free Ammonia (NH3) | The static table gives baseline logic; final value depends on entered values. |
Formula summary: Calculates free ammonia using the Emerson equation
Free NH3 by pH at 25°C/77°F (Common Reference Temperature)
| pH | TAN 0.5 ppm → NH3 | TAN 1.0 ppm → NH3 | TAN 2.0 ppm → NH3 | Risk Level |
|---|---|---|---|---|
| 6.0 | < 0.001 ppm | < 0.001 ppm | < 0.001 ppm | Safe |
| 6.5 | 0.001 ppm | 0.002 ppm | 0.004 ppm | Safe |
| 7.0 | 0.003 ppm | 0.006 ppm | 0.012 ppm | Safe |
| 7.5 | 0.010 ppm | 0.019 ppm | 0.038 ppm | Caution |
| 8.0 | 0.027 ppm | 0.053 ppm | 0.106 ppm | Dangerous |
| 8.2 | 0.042 ppm | 0.083 ppm | 0.166 ppm | Lethal |
| 8.5 | 0.076 ppm | 0.152 ppm | 0.304 ppm | Rapidly Lethal |
Values calculated at 25°C/77°F. Higher temperatures increase NH3 fraction by up to 30% at 30°C. Thresholds: Safe < 0.02 ppm, Chronic damage 0.02–0.05 ppm, Acute lethal > 0.05 ppm.
Troubleshooting Guide
1 Test kit reads 0.25–1.0 ppm TAN but fish look fine
Possible cause: Low pH or low temperature is keeping most TAN as harmless NH4+. NH3 is likely still below 0.02 ppm.
Use the calculator with your exact pH and temperature to confirm. If NH3 is under 0.02 ppm, monitor daily and continue cycling.
2 Fish gasping at the surface after a water change
Possible cause: Water change raised pH suddenly, converting stored NH4+ into toxic NH3. Classic Old Tank Syndrome trigger.
Immediately dose Seachem Prime for the full tank volume. Do a second 30% water change with slightly acidic water. Test TAN and pH every 30 minutes until fish recover.
3 Ammonia reading climbed even though I added more fish slowly
Possible cause: Biofilter is underpowered for new bioload. TAN is rising faster than bacteria can process it.
Dose Prime daily, reduce feeding by 50%, and test NH3 (not just TAN) using this calculator twice daily. If NH3 exceeds 0.05 ppm, do emergency 50% water change.
4 Ammonia reads 0 but fish show gill damage symptoms
Possible cause: Previous NH3 spike caused cumulative gill damage before you tested. Damage can persist even after ammonia drops to zero.
Increase surface agitation (oxygenation), maintain pristine water quality with NH3 below 0.01 ppm, and consider methylene blue dip for affected fish.
Glossary of Terms
- TAN
- Total Ammonia Nitrogen. The value measured by standard liquid test kits. Includes both toxic NH3 and harmless NH4+.
- NH3
- Free Ammonia / Unionized Ammonia. The toxic form of ammonia that passes through gill tissue and causes organ damage in fish.
- NH4+
- Ammonium. The ionized, relatively harmless form of ammonia. Dominates in acidic water (pH below 7).
- pKa
- The acid dissociation constant for ammonium (~9.25 at 25°C). Used to calculate the NH3/NH4+ ratio at any given pH and temperature.
- Old Tank Syndrome
- A condition where long-established tanks develop very low pH and high TAN. A sudden water change raises pH, instantly converting stored NH4+ into toxic NH3.
- Nitrogen Cycle
- The biological process where beneficial bacteria convert toxic ammonia (NH3/NH4+) into nitrite, then into relatively harmless nitrate.
- Unionized Ammonia
- Alternative name for Free Ammonia (NH3). 'Unionized' means the molecule carries no electrical charge, allowing it to cross biological membranes directly into fish tissue. Used interchangeably with NH3 in scientific literature.
- Nitrification
- The two-stage biological process in aquarium filters: first Nitrosomonas bacteria oxidize ammonia (NH3/NH4+) into nitrite (NO2-), then Nitrobacter bacteria convert nitrite into relatively harmless nitrate (NO3-).
- Beneficial Bacteria
- Colonies of nitrifying bacteria (primarily Nitrosomonas and Nitrosospira) that establish on filter media, substrate, and decorations. They form the biological filter that converts toxic ammonia into nitrite, then into relatively harmless nitrate. Destroying them (bleach, boiling, certain medications) causes 'New Tank Syndrome' even in established aquariums.
Scientific References
- Emerson, K. et al. (1975). Aqueous Ammonia Equilibrium Calculations. Journal of the Fisheries Research Board of Canada. (DOI: 10.1139/f75-234)
- Randall, D.J. & Tsui, T.K.N. (2002). Ammonia toxicity in fish. Marine Pollution Bulletin, 45(1-12), 17–23. (DOI: 10.1016/S0025-326X(02)00132-X)
- US EPA (1999). Ambient Water Quality Criteria for Ammonia – Freshwater.