What is the difference between ammonia NH3 and ammonium NH4+?
NH3 and NH4+ are two forms of the same dissolved ammonia pool, but they differ in charge and toxicity. Ammonia NH3 is un-ionised and crosses biological membranes more readily; ammonium NH4+ carries a positive charge. The two forms continuously interconvert in water. Their proportions depend mainly on pH and temperature: at the same total ammonia concentration, warmer or more alkaline water generally contains a larger NH3 fraction.
What does a water test actually measure?
Most routine tests do not directly separate NH3 from NH4+. They measure their combined pool, called total ammonia nitrogen or TAN, and the NH3 fraction is then estimated from pH and temperature. Some devices instead claim to measure “free ammonia”. Read the instructions, analytical method and measurement range rather than relying on the word “ammonia” on the label.
Check the unit as well. A result in mg N/L reports the mass of nitrogen, whereas mg/L NH3 or mg/L NH3 + NH4+ reports a chemical species. The numbers are not interchangeable without conversion. For long-term monitoring, retain the exact analyte name, unit, method, pH, temperature, time and sampling point with every result.
The FAQ on measuring ammonia in water introduces common testing methods. When comparing a kit, a photometer and a laboratory report, first confirm that all three express the same quantity.
Why do pH and temperature change the risk?
The equilibrium can be summarised as follows: NH3 accepts a proton to become NH4+, while NH4+ can lose one to become NH3. As pH rises, proportionally fewer protons are available and the equilibrium shifts towards NH3. Emerson and colleagues’ calculations also show that the un-ionised fraction increases with temperature. Salinity has an effect too, so a freshwater table should not be applied to marine or brackish water without verification.
This relationship means a stable TAN does not necessarily mean stable risk. A daytime pH rise in an algae-rich pond, or warming water, can increase the NH3 fraction even when the measured total has not changed. Conversely, a TAN result that is mostly NH4+ should not be dismissed: it identifies a nitrogen load that may exceed biofilter capacity and can become more hazardous if conditions change.
The toxicity of NH3 ammonia in freshwater is generally much greater in acute exposure than that of NH4+, but no single threshold guarantees safety for every fish. Species, life stage, exposure duration, temperature, pH, oxygen and other stressors all alter the response. The US EPA criteria are designed to protect freshwater communities, including sensitive invertebrates; they are not a universal aquarium instruction or a legal limit outside their regulatory context.
How can a result be interpreted correctly?
Use this order:
- Identify whether the test reports TAN, NH3 alone or another expression of ammonia.
- Record the exact unit and whether the value is expressed “as N”.
- Measure pH and temperature at the same time; include salinity for brackish or marine water.
- Use only a table or calculation designed for those units and that salinity.
- Compare the value with the same system’s history and the requirements of its species, not with an isolated number found online.
A colourimetric reading near the detection limit should be repeated with valid reagents and, when the consequences matter, confirmed using an appropriate method. Some conditioners, treatments and saline matrices interfere with common methods. Record every product added and provide that information to the laboratory or veterinarian.
What should you do when ammonia is detected?
Look first for excess production or inadequate removal: overfeeding, decomposing waste, a dead animal, high stocking density, an immature or disrupted biofilter, reduced flow, low oxygen or depleted alkalinity. Measure nitrite and dissolved oxygen as well. The FAQ on priority water parameters for a sick fish explains how to document the wider picture.
Do not try to “make ammonia safe” by abruptly lowering pH. A rapid acid-base shift can itself injure fish and destabilise the system. Depending on the setting, temporarily reducing feed, removing accessible organic matter, restoring aeration and arranging a compatible water exchange may reduce exposure. Any correction must account for the species, volume, replacement-water quality and biofilter function.
Rapid breathing, loss of appetite, marked lethargy, loss of balance or mortality requires prompt action. Vetofish can help interpret results and arrange suitable diagnostic testing; bring the original values and units, pH, temperature, sampling times, products used and a timeline of signs.