The newly-added fish (tank will not begin cycle until fish or some other source of ammonia is added) have not yet produced any measurable amount of ammonia. All readings should be near zero.
Ammonia level rises rapidly to near-dangerous; break-in fish may show some signs of discomfort, such as rapid breathing, poor feeding and darting about. First stage bacteria are beginning to become established, but their numbers are low at this point.
Ammonia level reaches its peak and begins to decline as first stage bacteria begin to convert it into nitrite. Nitrite levels begin to rise. Weaker fish may succumb to ammonia poisoning; a partial water change will dilute ammonia and reduce stress.
First stage bacteria are well established and are processing ammonia as fast as the fish are producing it. Ammonia level returns to zero and nitrites climb rapidly. Fish may perk up due to temporary reprieve from stressful conditions.
Nitrite levels reach their peak. Fish may again show some signs of distress. Small water changes (up to 20%) can help protect break-in fish without significantly prolonging cycling time.
Nitrite levels continue to be elevated for several weeks as the second stage bacteria begin turning nitrite into nitrate.
Second stage bacteria multiply rapidly and begin to catch up with built up nitrite. Nitrite levels drop rapidly and nitrate level becomes detectable.
Both first and second stage bacteria are now well established and can keep up with the ammonia output of the fish, turning it first into nitrite and then immediately into nitrate. Tank is now “cycled” and ready for additional livestock.
For a more detailed explanation, see New Tank Syndrome.