Ammonia poisoning is one of the biggest killers of aquarium fish. It occurs most often when a tank is newly set up. However, it can also occur in an established tank when too many new fish have been added at one time, when the filter fails, or if bacterial colonies die off due to the sudden change in water conditions.

Symptoms: 
Ammonia poisoning can happen suddenly, or over a period of days. Initially the fish may be seen gasping at the surface for air. The gills will begin to turn red, and may appear to be bleeding. The fish will lose appetite and become lethargic. In some cases fish may be laying at the bottom of the tank with clamped fins.

As the damage from the ammonia poisoning continues, there will be red streaks or bloody patches that appear on the body and fins. Internal damage is occurring to the brain, organs, and central nervous system. The fish begins to haemorrhage, and eventually dies.

Treatment: 
·  Lower pH below 7.0
·  25 - 50% water change
·  Use chemical to neutralize ammonia
·  Discontinue or reduce feeding

If the ammonia level rises above 1 ppm as measured by a standard test kit, begin treatment immediately. Lowering the pH of the water will provide immediate relief. Several water changes within a short period of time may be required to drop the ammonia to below 1 ppm.

Feedings should be restricted so that additional waste is reduced. No new fish should be added until the tank until the ammonia and nitrite levels have fallen to zero.

Because ammonia toxicity is linked to the pH, testing of both ammonia and pH levels are critical. Ammonia becomes increasingly toxic as the pH rises above 7.0.

At a level of level of 1 ppm or 1 mg/l, fish are under stress, even if they don't appear in acute distress. Levels even lower than that can be fatal if the fish are exposed continuously for several days. For that reason it is critical to continue daily testing and treatment until the ammonia drops to zero. When ammonia is elevated for a long period, it is not unusual to lose fish even after the ammonia levels start to drop.

Prevention: 
·  Stock new tanks slowly
·  Feed sparingly and remove uneaten food
·  Change water regularly
·  Test water regularly to catch problems early

The key to avoiding fish death from ammonia poisoning is to avoid ammonia spikes in the first place. When starting a new tank, add only a couple of fish initially and do not add more until the tank is completely cycled. Even in a well established tank, only add a couple of new fish at a time and avoid overstocking.

Feed fish small quantities of foods, and remove any food not consumed in five minutes.

Clean the tank weekly, taking care to remove any dead plants or other debris. Perform a partial water change at least every other week, more often in small heavily stocked tanks.

Test the water for ammonia at least twice a month to detect problems before they become serious.

Anytime a fish appears to be ill, test for ammonia to rule out ammonia poisoning.

If the filter stops, test for ammonia twenty-four hours later to ensure that the bacterial colonies that eliminate wastes were not affected.