posted 11-17-2003 11:59 AM         

A tank "cycle" is the breakdown of ammonia in you tank by bacteria. Fish waste and excess food are the primary cause of ammonia which is broken down into nitrite by bacteria. Ammonia and nitrite are both toxic to fish. The nitrite is then broken down to nitrate - which is relatively harmless. The "cycle" consists of the establishment of these colonies of bacteria in the aquarium. If testing for all 3, you will initially see a spike in ammonia. As ammonia reducing bacteria form, that spike will drop and a spike in nitrite will happen. Again, nitrite reducing bacteria will flourish and this spike will drop as nitrate increases. Nitrate can also be biodegrade, but it requires anoxic bacteria requiring deep gravel/sand beds etc. For most people the nitrate is best removed through weekly water changes of 20-25%.