Pond Ammonia & Find the Toxic NH₃
Converts TAN → NH₃
Total ammonia is mostly harmless ammonium, but the toxic un-ionized NH₃ share climbs steeply past pH 8 — about 5% at pH 8.0, 25 °C but ~36% at pH 9.0. Enter your TAN, pH and temperature to get the NH₃-N concentration versus your species' safe limit.
Enter your pond water test
Next: reduce the toxic NH₃: halve the feed for 24–48 h and exchange some water. Dropping pH by 0.94 (to ≤ pH 7.56) or TAN by 86.2% (to ≤ 0.276 mg/L) brings NH₃-N back under 0.05 mg/L.
NH₃ fraction = 1 / (1 + 10^(pKa − pH)); pKa = 0.09018 + 2729.92/T(K) (Emerson et al. 1975). NH₃-N = TAN × fraction. Species safe limits after Boyd, Water Quality for Pond Aquaculture / SRAC. Planning estimate — confirm with a meter and test at the pH peak.
Pond ammonia — key facts
- NH₃ fraction
- 1 ÷ (1 + 10^(pKa − pH))
- pKa
- 0.09018 + 2729.92 ÷ T(K)
- NH₃-N
- TAN × fraction (mg/L)
- pH 8.0, 25 °C
- ≈ 5.4% un-ionized
- pH 9.0, 25 °C
- ≈ 36% un-ionized
- General safe
- ≈ 0.05 mg/L NH₃-N
- Most toxic form
- NH₃ (gill-permeable)
- Privacy
- Runs in your browser; nothing uploaded
Un-ionized ammonia fraction (%) by pH and temperature
Percent of total ammonia present as the toxic NH₃ form, computed from the Emerson et al. (1975) equilibrium. Read across to your water temperature, down to your pH.
| pH | 16 °C | 20 °C | 24 °C | 28 °C | 32 °C |
|---|---|---|---|---|---|
| 6.5 | 0.09% | 0.13% | 0.17% | 0.22% | 0.29% |
| 7.0 | 0.29% | 0.39% | 0.53% | 0.69% | 0.91% |
| 7.5 | 0.92% | 1.24% | 1.64% | 2.16% | 2.83% |
| 8.0 | 2.86% | 3.81% | 5.02% | 6.54% | 8.42% |
| 8.5 | 8.51% | 11.12% | 14.31% | 18.12% | 22.53% |
| 9.0 | 22.73% | 28.36% | 34.56% | 41.16% | 47.91% |
| 9.5 | 48.19% | 55.59% | 62.55% | 68.87% | 74.41% |
Source: Emerson K. et al. (1975) J. Fish. Res. Board Can. 32:2379. Bold = above ~10% un-ionized.
Species safe limits for un-ionized ammonia (NH₃-N)
| Species | Safe (mg/L) | Stress (mg/L) | Acute (mg/L) | Note |
|---|---|---|---|---|
| Tilapia (warm, tolerant) | 0.10 | 0.20 | 2.00 | Among the most ammonia-tolerant cultured fish. |
| Channel catfish | 0.05 | 0.12 | 1.00 | Pond standard; sustained NH3-N below 0.05 mg/L. |
| Common carp | 0.07 | 0.15 | 1.20 | Moderately tolerant warm-water cyprinid. |
| Rohu / Indian major carp | 0.06 | 0.13 | 1.00 | Composite carp polyculture species. |
| Pangasius (striped catfish) | 0.10 | 0.20 | 1.50 | Air-breather; tolerant in intensive ponds. |
| Shrimp (penaeid, brackish) | 0.03 | 0.07 | 0.45 | Sensitive; keep NH3-N very low for postlarvae. |
| Rainbow trout (cold) | 0.02 | 0.05 | 0.30 | Cold-water; very ammonia-sensitive. |
| Atlantic salmon (cold) | 0.02 | 0.04 | 0.25 | Most sensitive; smolt stage especially. |
| General aquaculture limit | 0.05 | 0.10 | 0.50 | Conservative default (~0.05 mg/L NH3-N). |
Source: Boyd, Water Quality for Pond Aquaculture; SRAC publications. Chronic safe = long-term, no growth effect.
A total-ammonia reading alone cannot tell you the risk
Ammonia builds up in ponds from fish excretion and the breakdown of uneaten feed. A test kit reports total ammonia-nitrogen (TAN), but that figure mixes two very different molecules: ammonium (NH₄⁺), which fish tolerate well, and un-ionized ammonia (NH₃), which crosses the gill membrane and is highly toxic. The split between them is set by pH and temperature through the ammonia ionization equilibrium — so the same TAN can be perfectly safe or lethal depending on conditions.
This tool converts your TAN to the toxic NH₃-N using the Emerson equation, then compares it to the safe, stress and acute limits for your species. Because the relationship is logarithmic in pH, a one-point pH rise multiplies the toxic share roughly ten-fold — which is why afternoon algal pH spikes are so dangerous. Pair it with the Pond Oxygen Budget tool: low oxygen and high ammonia are the two leading causes of pond fish kills.
How to use it — 5 steps
- 1Test mid-afternoon. Measure total ammonia (TAN), pH and temperature when pH peaks.
- 2Enter the readings. Type TAN in mg/L, the pH and the water temperature.
- 3Pick your species. Select the cultured species to load its safe and acute limits.
- 4Read the NH₃-N. See the un-ionized NH₃-N and the toxic fraction versus the limit.
- 5Act on the verdict. If unsafe, apply the pH drop or TAN cut shown — exchange water, cut feed, manage pH.
Frequently Asked Questions
How do you calculate un-ionized ammonia from total ammonia?+
Total ammonia-nitrogen (TAN) splits between non-toxic ammonium (NH₄⁺) and toxic un-ionized ammonia (NH₃). The toxic fraction is 1 ÷ (1 + 10^(pKa − pH)), where pKa = 0.09018 + 2729.92 ÷ T(Kelvin) from Emerson et al. 1975. Multiply that fraction by your TAN to get NH₃-N in mg/L. For example, TAN 2 mg/L at pH 8.0 and 25 °C gives a fraction of about 5.4%, so 0.11 mg/L NH₃-N.
Why does the toxic ammonia rise so fast with pH?+
The NH₄⁺/NH₃ equilibrium is logarithmic in pH, so each one-unit rise in pH multiplies the un-ionized share roughly ten-fold over the relevant range. At pH 7.0, 25 °C only about 0.6% of ammonia is the toxic NH₃ form; at pH 8.0 it is ~5.4%; at pH 9.0 it is ~36%. That is why a single afternoon pH spike from algal photosynthesis can turn a safe ammonia level dangerous.
What is a safe un-ionized ammonia level for fish?+
A widely used chronic safe limit is about 0.05 mg/L NH₃-N for general aquaculture and channel catfish, with tolerant warm-water species like tilapia and pangasius safe to roughly 0.10 mg/L, and sensitive cold-water species (trout, salmon) and shrimp postlarvae needing much lower levels (0.02–0.03 mg/L). Acute mortality risk begins well above these — often above 0.3–2.0 mg/L depending on species.
Does temperature affect ammonia toxicity?+
Yes. Warmer water lowers the pKa, so a larger share of total ammonia is the toxic un-ionized form at the same pH. At pH 8.0 the NH₃ fraction rises from about 4.0% at 20 °C to about 7.5% at 30 °C. Warm summer ponds therefore carry more ammonia risk for the same TAN reading.
Is the ammonia in my pond toxic right now?+
Enter your total ammonia (TAN), pH and water temperature and pick your species. The tool returns the un-ionized NH₃-N concentration and compares it to that species' safe and acute limits, giving a safe, stress, or acutely toxic verdict. If it is above the safe limit it also tells you how far pH or TAN must drop to clear it.
When should I test pond ammonia?+
Test total ammonia together with pH in the mid-to-late afternoon, when algal photosynthesis pushes pH (and therefore the toxic NH₃ fraction) to its daily peak. A reading that looks safe at dawn — when pH is lowest — can be in the stress range by mid-afternoon, so always check at the pH peak.
What lowers ammonia toxicity quickly?+
The fastest levers are a partial water exchange (replace 20–30% of pond volume to dilute TAN) and stopping or cutting feed for 24–48 hours, since feed is the main ammonia source. Reducing afternoon pH (by managing the algal bloom and avoiding liming when pH is high) cuts the toxic fraction directly, and aeration supports the nitrifying bacteria that convert ammonia onward to nitrite and nitrate.
What is the difference between TAN, NH₃ and NH₄⁺?+
Total ammonia-nitrogen (TAN) is the sum of the two forms a test kit measures together: ammonium ion (NH₄⁺), which is relatively non-toxic, and un-ionized ammonia (NH₃), which is highly toxic to fish. Only NH₃ crosses the gill membrane easily and harms the fish, so converting TAN to NH₃ via pH and temperature is essential — a TAN reading alone cannot tell you the risk.
How much do I need to lower pH or TAN to be safe?+
The calculator back-solves both: it finds the pH that would put NH₃-N exactly at your species' safe limit (holding TAN and temperature) and the TAN that would do the same (holding pH and temperature). It then reports the pH drop and the percent TAN cut required. Often a small pH reduction is enough because the relationship is so steep.
Does this work for aquaponics and tanks too?+
Yes — the ammonia ionization chemistry is identical in tanks, recirculating systems and aquaponics. Enter the same TAN, pH and temperature. Aquaponic systems often run at lower pH for nutrient availability, which conveniently keeps the toxic NH₃ fraction low even when TAN is present during cycling.
Is 1 mg/L of total ammonia dangerous?+
It depends entirely on pH and temperature. At pH 7.0 and 24 °C, 1 mg/L TAN is only about 0.005 mg/L NH₃-N — safe for most species. At pH 9.0 and 30 °C the same 1 mg/L TAN is over 0.4 mg/L NH₃-N — acutely toxic for sensitive species. This is exactly why the un-ionized conversion matters and a single TAN number can mislead.
Why does this differ from a pond oxygen-budget tool?+
The oxygen-budget tool tracks dissolved oxygen falling overnight; this tool tracks the chemical equilibrium that turns measured total ammonia into the toxic un-ionized NH₃ form against species limits. Both are core pond water-quality checks but answer different questions — low oxygen and high ammonia are the two most common causes of pond fish kills.