The nitrifiers also known as the good or beneficial bacteria, which are present after successfully cycling a new tank, providing biological filtration, without which our tanks would turn into a "toxic waste dump".
Let's take a closer look at our little friends:
Nitrifiers consist of two bacteria species. Both separate, but dependent. While the first strain settles as soon as ammonia is present, the second one settles as soon as nitrites are present.
Nitrifiers rely on a biochemical reaction (oxidation) by the means of using oxygen as a way of transporting electrons, drawn from the ammonia/ nitrite compounds. Just imagine oxygen as a garbage collector.
Under low oxygen levels, the bacteria use nitrite/ nitrate as an acceptor, or garbage collector, reversing the process from nitrifying to denitrifying (removal of nitrates).
As an aquarium is loaded with oxygen, the bacteria need ammonia in order to stay alive.
Fish respiration and decaying food provides ammonia in sufficient amounts to ensure the proper function. So in theory: without ammonia, the bacteria will enter a resting phase without loosing much of their energy, ready to get to work again once ammonia is available.
Certain conditions have to be provided to promote and allow this process.
Nitrifiers require oxygen: 4.5 milligrams (equal to ppm) of dissolved oxygen for every milligram of nitrogen transformed; or oxidized to use the correct term. Oxygen is of ample supply in the aquarium.
Nitrifiers prefer a pH level from 7 - up to mid 8's. However, a stable pH level with no fluctuations is more important. This also relates to most fish kept in an aquarium. Fish can take pH levels slightly out of their preferred range, but not the fluctuation. The goal in either case is a stable pH.
As for temperature, the bacteria prefer a range from 65 - 85 degrees Fahrenheit. The activity of the bacteria will progressively decline as the temperature changes.
Life threatening conditions are reached at 95 degrees; adding just a few degrees and the activity comes to a total stop. As the preferred temperature range corresponds to most fish keeping requirements, temperature is of less concern.
Also of lesser concern is the salinity (salt content of the water). Nitrifiers are adaptive to a sizable range of salinity, satisfying all salt-water species. Interesting in this case is that the bacteria can adjust to changes in salinity without loosing their activity. Interesting, because you can switch from freshwater to marine (or vice versa) while transferring the existing bacteria to the new tank. This also allows seeding marine tanks with water from a freshwater aquarium for cycling purposes.
Nitrifiers are light sensitive, especially toward ultraviolet (UV/ sunlight). Room light has a negative impact on bacterial activity as well. Colonizing the filter is therefore the preferred settlement of the bacteria, as it provides a dark environment. Light exposure (i.e. cleaning the filter) will not cause stress, as the time frame is too short allowing the colony to recuperate within hours.
The nitrifier's colony creates a surrounding, slimy bio-film, as they clutter together. This somewhat protects the settlement from light exposure. Good films smell earthy, if otherwise, it is an indication of problems in the aquatic environment.
It goes without saying that anti-bacterial (antibiotic) medication will have an impact on the nitrifying bacteria. The same applies to heavy metals. The interference might go unnoticed, but can also yield a detrimental effect.