After the bacteria have divided several times they use up their energy reserves and they fail to multiply. Also a bacterial bloom can occur when the gravel bed is disturbed if it is not cleaned routinely. A biofilm establishes and develops on the gravel and surrounds each piece of gravel. Various types of bacteria form within this film and can become trapped and deprived of the proper nutrients and oxygen. Many times the filter itself becomes clogged thus reducing water flow and oxygen.
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.