OK a short summary then...
Code:
Fishes ---> Output Ammonia
Ammonia---> Provides food for some bacteria ----> breaks down to Nitrite
Nitrite---> Provides food for another type of bacteri ---> which results in the more harmless nitrate.
As you can imagine it takes time for the bacteria colony to get large enough to handle the amount of ammonia and nitrite being produced. By using old filter media, it is assumed that the bacteria colony there is already established (lots of bacteria) and thus able to break down the ammonia and nitrites more effectively. For more info, search the web for information on the nitrogen cycle.
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