Anaerobic bacteria form and begin to decompose waste products. This anaerobic decomposition produces hydrogen sulfide. When cleaned,or disturbed, these bacteria, which were dormant, now begin to multiply in the presence of oxygen and nutrients and releases hydrogen sulfide. Hydrogen sulfide is released from gas pockets in the gravel, it is then absorbed by the gills and combines with the red blood cells of fish. Then the red blood cells cannot carry oxygen to the body and the fish suffocates.
Nitrifying bacteria are bacteria that breakdown organic waste into simple compounds.
Most of this breakdown is the work of bacteria that convert the waste products to ammonia. This allows for the nitrogen cycle to take place. When this cycle has occurred your tank has established. Nitrifying bacteria breakdown feces and waste into ammonia. From this point other bacteria use this ammonia to break it down further into nitrites which are very toxic.