From fresh aquarium
"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 due to power or mechanical failure, or if bacterial colonies die off due to the use of medications or sudden change in water conditions.
Symptoms
# Fish gasp for breath at the water surface
# Purple or red gills
# Fish is lethargic
# Loss of appetite
# Fish lays at the bottom of the tank
# Red streaking on the fins or body
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 or lilac in color, and may appear to be bleeding. The fish will being to lose its appetite and become increasingly lethargic. In some cases fish may be observed laying at the bottom of the tank with clamped fins.
As the damage from the ammonia poisoning continues, the tissues will be damaged as evidenced by 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 hemorrhage internally and externally, and eventually dies. "