Ammonia Toxicity in Fish

TL;DR. Ammonia toxicity is a life-threatening aquatic emergency caused by the accumulation of toxic fish waste in the water, leading to severe gill damage, suffocation, and brain damage.

Gasping at the water surface is a cardinal sign of respiratory distress caused by ammonia toxicity.

What is it?

Ammonia toxicity, scientifically known as environmental hyperammonemia, is one of the most common and devastating conditions affecting aquarium and pond fish. It occurs when ammonia—a primary waste product excreted by fish—accumulates in the water to dangerous levels. In a healthy, established aquatic ecosystem, beneficial bacteria continuously break down this waste. However, when this biological filter fails or has not yet established, ammonia levels spike, turning the fish's living environment highly toxic.

To understand ammonia toxicity, it is vital to understand the two forms of ammonia that exist in water: unionized ammonia (NH3) and ionized ammonia (ammonium, or NH4+). Ionized ammonia is relatively non-toxic to fish, whereas unionized ammonia is highly toxic. The balance between these two forms is dictated by the water's pH and temperature. As pH and temperature rise, a greater percentage of the ammonia shifts into the highly toxic unionized form. Because fish are aquatic ectotherms, much of our veterinary guidance relies on environmental manipulation and extrapolation from standard aquaculture and ornamental fish medicine principles.

When toxic unionized ammonia accumulates in the water, it easily crosses the fish's gill membranes. Once inside the body, it causes physical damage to the delicate respiratory tissues and enters the bloodstream. This leads to systemic poisoning, primarily targeting the respiratory and nervous systems. The chemical burns the gills, preventing the fish from absorbing oxygen, while the accumulation of ammonia in the blood crosses the blood-brain barrier, causing severe, irreversible neurological damage.