Un-ionized Ammonia (NH3) Toxicity

Un-ionized ammonia (NH3) is the lipophilic, freely diffusible fraction of total ammonia nitrogen that crosses gill membranes and blood-brain barriers; the ionized ammonium form (NH4+) is largely harmless. NH3 share rises with pH and temperature: at pH 7.0 and 20 C about 0.4 percent of TAN is NH3, at pH 8.0 around 5-6 percent, and at pH 9.0 more than 30 percent. Chronic safe levels are roughly 0.005-0.02 mg/L NH3 for sensitive species like trout and salmon, while warmwater fish such as tilapia and common carp tolerate 0.05 mg/L or more; acute lethal concentrations are 10-100 times higher. The fraction is calculated from measured TAN, pH, temperature, and salinity using published tables or online calculators. For farmers the operational lesson is that the same TAN reading becomes several times more dangerous after a pH rise, so management decisions must combine the two values. Damage shows as gill hyperplasia, mucus buildup, lethargy, and reduced feeding, with mortality often delayed 24-48 hours after exposure.