Ammonia Poisoning in Aquariums

Causes and symptoms of ammonia poisoning, how pH and temperature change its toxicity, immediate response, and how to prevent it.

What it is

Ammonia poisoning occurs when total ammonia accumulates in the water and damages fish. Ammonia exists as un-ionised NH3 and ionised ammonium NH4+; their sum is total ammonia nitrogen (TAN). The un-ionised NH3 form is roughly 100 times more toxic to fish than ammonium.

Causes

Ammonia builds up when waste production outpaces the biofilter: an uncycled new tank, an overstocked or overfed tank, a crashed filter, decaying matter, or interrupted bacteria after cleaning. It is the central problem behind new tank syndrome.

How pH and temperature change toxicity

The fraction of toxic un-ionised ammonia rises with higher pH and higher temperature, and decreases slightly with salinity. The same total ammonia reading is therefore far more dangerous in warm, alkaline water than in cool, acidic water.

Symptoms

Gasping at the surface or rapid gill movement
Red or inflamed gills
Lethargy and loss of appetite
Clamped fins and erratic or distressed swimming
Increased susceptibility to bacterial infection

Toxic levels

Damage to fish tissues can begin around 0.05 mg/L of un-ionised ammonia, and sensitive fish typically die near 2.0 mg/L. Because pH controls the un-ionised fraction, a given total ammonia value translates to very different NH3 exposure at different pH.

Immediate response

Stop or sharply reduce feeding
Perform partial water changes to dilute ammonia (25-50% in small systems)
Use a dechlorinator on replacement water; an ammonia-detoxifier can temporarily reduce toxicity
Maintain strong aeration and dissolved oxygen above 5 mg/L
Test ammonia and nitrite repeatedly until both read zero

Prevention

Fully cycle the tank before stocking
Stock gradually and avoid overstocking
Feed sparingly and remove uneaten food
Avoid disrupting the biofilter during maintenance
Test water regularly, especially in new tanks