ammonia and nitrite poisoning

found a couple of decent articles
http://freshaquarium.about.com/cs/disease/p/ammoniapoison.htm
http://freshaquarium.about.com/cs/disease/p/nitritepoison.htm

if we talk more about this then we would came to know Nitrite poisoning follows closely on the heels of ammonia as a major killer of aquarium fish. Just when you think you are home free after losing half your fish to ammonia poisoning, the nitrites rise and put your fish at risk again. Anytime ammonia levels are elevated, elevated nitrites will soon follow. To avoid nitrite poisoning, test when setting up a new tank, when adding new fish to established an tank, when the filter fails due to power or mechanical failure, and when medicating sick fish.
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.
And you know Ammonia is the most lethal form of chemical toxic to fish and aquatic plants. It will directly cause burns to the to the skin layer leading to redness, clamped fins and in worst cases, concentration of 0.2 – 0.5ppm is enough to kill all the fish in an aquarium. Ammonia poisoning is considered a common threat effecting newly setup aquarium whereby colony of beneficial bacteria has yet to establish. It is also sometimes referred to as “New Tank Syndrome” and aquarium owners often get shocked upon seeing all the fish die at the same time.