Originally Posted by Mebbid
In order for bacteria to die from lack of oxygen it would require that the oxygen levels reach near 0. If the oxygen level reaches too low of a level the bacteria will just stop its role in the nitrogen cycle and go dormant for a time in the same way it will if the pH drops too low.
If oxygen in water acted how you say it does then every time the power goes out for anyone for more than a day then they will have to completely restart their bacteria cycle not to mention buy an entire new tank full of fish because they would have all suffocated.
Sorry, but no. Locally the amount of oxygen can reach zero very quickly. Bacteria near the surface of the filter will be fine, bacteria in the tank will be unaffected (remember while the filter has a much higher bacteria per square inch of space, the substrate/decorations/glass/etc. harbors the majority of the beneficial bacteria in a seasoned tank), but the bacteria in the biological media will not be for very long.
What happens when the power goes back on after a day is a large amount of the beneficial bacteria in the filter will have died, but the bacteria in the tank will buffer during the repopulation period of the filters biological media. Because of this you will encounter a mini-cycle (or if lucky and have a heavily planted tank (where the bacterial biological filter is weak to begin with), or enough surface area in the tank due to using something like sand versus a bare-bottomed tank) you will never even see the effects of the power outage.
But to think you haven't significantly damaged the filter's inhabitants would be unwise.