Water Quality Calculators

The Chemistry of Ammonia in Ponds

Ammonia is the primary metabolic waste product of fish — excreted directly through the gills — and is also released by the decomposition of organic matter such as uneaten fish feed, dead algae, leaves, and bottom muck. In a healthy pond, nitrifying bacteria convert ammonia to nitrite and then to nitrate through a process called nitrification. When ammonia production exceeds the biological capacity to convert it, concentrations rise and the water becomes dangerous.

What makes ammonia management tricky is that the number on your test kit — total ammonia nitrogen, or TAN — doesn't tell you the whole story. TAN is the sum of two chemical forms that exist in equilibrium, and only one of them is toxic.

NH₃ vs. NH₄⁺: Two Forms, Very Different Dangers

In water, ammonia exists in two forms simultaneously: un-ionized ammonia (NH₃) and ionized ammonium (NH₄⁺). These two forms are in constant chemical equilibrium — shifting back and forth depending on water conditions.

This is why TAN alone is insufficient for assessing ammonia risk. A TAN reading of 2.0 mg/L could be perfectly safe or immediately lethal depending on pH and temperature — because those two variables determine how much of that 2.0 mg/L exists in the toxic NH₃ form versus the relatively benign NH₄⁺ form.

Why pH Is the Dominant Factor

The equilibrium between NH₃ and NH₄⁺ is governed by pH more than any other variable. At low pH (acidic conditions), the equilibrium shifts heavily toward NH₄⁺ — the ionized, less toxic form. At high pH (alkaline conditions), the equilibrium shifts toward NH₃ — the un-ionized, toxic form. Each full unit increase in pH roughly multiplies the fraction of free ammonia by 10.

This has profound practical implications. A pond with a TAN of 1.0 mg/L at pH 7.0 and 70°F has a free ammonia concentration of about 0.004 mg/L — well within safe limits. That same 1.0 mg/L TAN at pH 9.0 and 70°F yields approximately 0.29 mg/L of free ammonia — deep into the lethal range. The TAN didn't change. The danger did.

Temperature's Role

Temperature also shifts the NH₃/NH₄⁺ equilibrium, though less dramatically than pH. Warmer water increases the fraction of toxic free ammonia. This matters because ammonia problems most often occur in summer — when water is warmest, biological oxygen demand is highest, fish metabolism (and therefore ammonia excretion) peaks, and algae blooms and die-offs release nutrients. The combination of high temperature and elevated pH (which often rises in the afternoon due to photosynthesis consuming CO₂) creates a compounding risk window.

The table below illustrates how dramatically the percentage of total ammonia present as toxic NH₃ changes across common pond pH and temperature combinations:

pH	59°F (15°C)	68°F (20°C)	77°F (25°C)	86°F (30°C)
7.0	0.27%	0.40%	0.57%	0.81%
7.5	0.86%	1.24%	1.77%	2.52%
8.0	2.65%	3.83%	5.38%	7.52%
8.5	7.97%	11.18%	15.25%	20.29%
9.0	21.50%	28.47%	35.79%	43.63%

At pH 7.0, less than 1% of total ammonia is in the toxic form regardless of temperature. At pH 9.0, more than a fifth — and up to nearly half — is free ammonia. This is why pH management and monitoring are inseparable from ammonia management.

EPA Criteria: CMC and CCC

The U.S. Environmental Protection Agency publishes two ammonia criteria for the protection of aquatic life, both expressed as total ammonia nitrogen (TAN) and adjusted for pH and temperature:

CMC (Criterion Maximum Concentration) is the acute threshold — the highest TAN concentration that aquatic organisms can be exposed to briefly (one hour) without expectation of unacceptable effects. Exceeding the CMC means fish kills are likely.

CCC (Criterion Continuous Concentration) is the chronic threshold — the highest TAN concentration that can be maintained indefinitely (30-day rolling average) without harming aquatic life over time. Sustained levels above the CCC cause sublethal stress: reduced growth, impaired reproduction, increased disease susceptibility, and behavioral changes like reduced feeding.

The calculator above computes both thresholds at your specific pH and temperature, which is important because the EPA criteria are not fixed numbers. They shift with water chemistry — the same reason free ammonia toxicity shifts. When salmonids (trout, salmon, or char) are present, the CMC is calculated using a more protective formula that accounts for the heightened sensitivity of cold-water species to ammonia.

What Drives Ammonia Spikes in Ponds?

Understanding why ammonia rises helps prevent it from becoming a crisis. Common triggers include fish kills or large-scale organism die-offs that release stored nitrogen, overfeeding fish (uneaten feed decomposes rapidly), overstocking beyond the biological carrying capacity, sudden algae crashes that dump organic matter into the system, disruption of the nitrifying bacteria colony (from herbicide applications, rapid temperature changes, or oxygen depletion), and heavy organic loading from leaf litter, grass clippings, or fertilizer runoff.

In many cases, ammonia spikes are the result of a cascade: nutrient loading fuels algae growth, the algae crash depletes oxygen, low oxygen kills nitrifying bacteria, and ammonia accumulates without the biological conversion process to remove it. Aeration breaks this cycle at the most critical point — maintaining the dissolved oxygen that nitrifying bacteria need to do their job.

Managing Ammonia: A Proactive Approach

Reactive ammonia treatment — emergency water changes, chemical binders — is sometimes necessary, but the most effective management is proactive. The goal is to maintain a robust population of nitrifying bacteria and the dissolved oxygen they require.

Aeration

Adequate aeration is the single most important factor in ammonia management. Nitrifying bacteria (Nitrosomonas and Nitrobacter) are obligate aerobes — they cannot function without oxygen. Every 1.0 mg/L of ammonia oxidized to nitrate consumes approximately 4.6 mg/L of dissolved oxygen. In a pond with even moderate ammonia loading, the oxygen demand of nitrification alone can exceed what the pond produces naturally. Supplemental aeration ensures the oxygen is always there.

Beneficial Bacteria

Commercial nitrifying bacteria products accelerate the establishment and recovery of the nitrogen cycle, particularly after disruptions. Regular application throughout the warm season maintains bacterial populations at levels capable of processing the ammonia load.

Reduce Nutrient Loading

Every management tool works better when the incoming nutrient load is controlled. Reduce or eliminate fish feeding during high-risk periods, prevent fertilizer runoff from reaching the pond, manage waterfowl populations, and use phosphorus-binding products like MetaFloc to limit the nutrient base that drives the entire cycle.