Ammonia Poisoning in Aquariums

Ammonia poisoning is an environmental toxicity from unionized NH3 in an incomplete nitrogen cycle, damaging gills and brain. Toxicity rises with pH and temperature.

Overview

Ammonia poisoning is an environmental rather than infectious disease, caused by toxic unionized ammonia (NH3) accumulating in water with an incomplete or disrupted nitrogen cycle. Damage can be detectable from as little as 0.25 mg/L total ammonia. The ratio of NH3 (toxic) to NH4+ (much less toxic) increases sharply with rising pH and temperature, so the same total ammonia reading is more dangerous in warm alkaline tanks than in cool acidic ones.

Symptoms

Red, bleeding or purple gills
Gasping at the water surface
Darkening of body coloration
Lying on the bottom, refusing to swim
Red streaking on fins or body
Frayed fins and chemical burns in severe cases
Sudden death in acute exposure

Causes

Most common scenarios: a tank stocked before the nitrogen cycle has established ('new tank syndrome'); loss of biological filtration after filter cleaning with tap water, equipment failure or extended power outage; severe overstocking; overfeeding; large amounts of dead organic matter; and the introduction of fish to chloramine-treated tap water without dechlorinator (chloramine releases ammonia).

Diagnosis

Diagnosis is by water test: a liquid reagent kit detects total ammonia, which can then be split into NH3/NH4+ ratios using a pH-temperature table. Combined with symptoms such as gill damage and surface gasping, any ammonia reading above 0 mg/L in a stocked tank is treated as toxic.

Treatment

Step 1: Quarantine and reduce exposure

Immediately perform a 50% water change with dechlorinated water at matched temperature. Stop feeding for 24-48 hours to reduce nitrogenous load. Increase aeration — damaged gills compensate poorly for low oxygen. Do not clean filter media at this stage.

Step 2: Detoxification

Dose an ammonia binder containing sodium hydroxymethanesulfonate (e.g. Seachem Prime) per package, which converts NH3 to non-toxic NH4+ for approximately 24-48 hours.
Re-test ammonia every 12-24 hours and repeat dosing/water changes until ammonia is undetectable.
Add a live nitrifying bacteria culture (e.g. Tetra SafeStart, Stability) to seed or re-seed biological filtration.
Where available, add filter media from a healthy, fully cycled tank to accelerate recovery.
Where pH is high, reducing it by 0.2-0.4 units to the safe range for the species shifts equilibrium toward less toxic NH4+; do not pursue this if it stresses sensitive species.

Step 3: Recovery and re-cycling

Continue daily water changes and detoxifier dosing until ammonia and nitrite both read 0 mg/L. Re-cycling typically takes 1-2 weeks with bacterial supplementation, longer without. Resume feeding gradually with small amounts of high-quality food. Monitor gills daily — fish with chemical burns may need 2-4 weeks to fully heal.

Prevention

Fully cycle the tank before adding fish
Do not overstock — research adult sizes of all species
Do not overfeed; remove uneaten food
Test ammonia weekly and after any change to filtration or stocking
Preserve filter media: rinse only in tank water, never tap water
Always use a dechlorinator that also neutralizes chloramine