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Marine Aquaria and
Miniature Reefs

Dynamic Aquaria : Building
Living Ecosystems

Aquarium Plants: The
Practical Guide

Handbook of Fish Diseases

The Nitrogen Cycle

The nitrogen cycle is the most important principle that "you" need to understand
concerning an aquarium environment. In nature, the fish enjoy what is called an 
"open" type system due to the very large water volume to fish ratio that they live 
in. This allows their water to be constantly renewed. In your aquarium, however, a
"closed" type system exists where, without action on your part such as changing 
the water, their water will never be renewed and needs to be filtered properly for 
them to remain healthy. Proper filtering consists of 3 types, mechanical, chemical, 
and biological. The most essential of the 3 is the biological filtering that is 
accomplished by the nitrogen cycle, and is described below.

The cycle begins when you add fish to the aquarium. Between excess uneaten 
decayed food and waste that is generated by these fish, toxic ammonia is formed. 
(At pH levels of 7.0 and above, ammonia irritates the gill tissues of fish, and even at
moderate levels, can cause death. At pH levels below 7.0, ammonia is present in the
form of ammonium, and is much less toxic.) These ammonia levels will increase for
about 2 weeks until aerobic bacteria called "nitrosomonas" grow to sufficient quantities 
in the filter to convert the ammonia to toxic nitrite. (Nitrite destroys the hemoglobin in 
the fish's blood and eventually prevents the blood from carrying oxygen) As this 
happens, the ammonia levels will quickly begin to drop as the nitrite levels slowly
increase. These nitrite levels will continue to increase for about 2 weeks until aerobic
bacteria called "nitrobacters" grow to sufficient quantities in the filter to convert the 
nitrite to much less toxic nitrate. Again, as the nitrite levels quickly decrease, the 
nitrate levels will slowly increase. Once your tank has reached this point (about 5-6
week's total), it is said to have "cycled".

Nitrate is harmless except in higher levels (such as above 40 parts per million) which 
will promote algae growth in the tank. Even higher levels can result in stress that can
weaken the immune system of the fish and make them susceptible to disease. All 
you need to do now, is to perform your regular partial water changes in order to keep 
a moderately low nitrate level. If this practice is followed routinely, you should have no
problems maintaining your biological filter.


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