For oxidation of ammonia to ...

For oxidation of ammonia to nitrite then nitrate, you need 3 things.  

1) The correct bacteria.  These need to be replaced if they are lost from a plant.  They have to be freshly made and they lose their viability on storage.  These are not the same as freeze-dried bacteria.

2) Free oxygen throughout the plant because the bacteria which oxidise are strict aerobes

3) bicarbonate.  Because the bacteria fix their carbon from carbonate, if that becomes depleted then they are unable to grow.

Loss of oxygen inhibits the growth of the bacteria but they will recover if oxygen is only lost for a short time.