I get a lot of shops ask me about nitrite spikes where there is no ammonia or nitrate present... boggles the mind! There is a scientific explanation and I think it is mostly the overuse of moving beds in sumps / LSS promoting what is called facultative anaerobic bacteria -
A facultative anaerobe is an organism that can live and grow in both the presence and absence of oxygen. These organisms are able to switch between aerobic respiration (using oxygen as the final electron acceptor in the electron transport chain) and anaerobic respiration or fermentation (using other molecules as electron acceptors).
Facultative anaerobes are typically able to grow faster and produce more energy in the presence of oxygen, as aerobic respiration is a more efficient way to produce ATP (adenosine triphosphate) than anaerobic respiration. In the absence of oxygen, facultative anaerobes are able to continue growing by using other electron acceptors, such as nitrate NO3.
This ability to adapt to changing oxygen conditions allows them to survive in a wide range of environments, from the oxygen-rich surface layers of soil to the anaerobic sediments at the bottom of a lake or pond.
An interesting quirk with too much facultative anaerobic bacteria in fish systems are Nitrite spikes from nitrate to nitrite and ammonia to nitrite. Yes you read that correctly - traditionally we think of nitrite NO2- being converted to Nitrate NO3- during nitrification. However the school of thought is if you have too much aerobic bacteria or facultative anaerobic bacteria this can 'pinch' (scientific word) a valency off the 3 on the NO3 (and thus it is now NO2-) and cause denitrification, which is the conversion of nitrate (NO3-) to nitrogen gas (N2) occurring through a series of complex chemical reactions that involve several types of bacteria. These reactions can be summarised as follows:
Nitrate (NO3-) is first converted to nitrite (NO2-) by nitrate-reducing bacteria.
Nitrite (NO2-) is then converted to nitric oxide (NO) by nitrite-reducing bacteria.
Nitric oxide (NO) is converted to nitrous oxide (N2O) by nitric oxide-reducing bacteria.
Nitrous oxide (N2O) is finally converted to nitrogen gas (N2) by nitrous oxide-reducing bacteria.
These reactions involve a series of enzymatic steps that require energy and reducing agents, such as organic matter, to drive the process.
It's worth noting that denitrification is a complex process and there are many factors that can influence its efficiency, including temperature, pH, oxygen levels, and the availability of carbon and other nutrients. In aquatic systems, it's important to monitor nitrate and nitrite levels to ensure that they remain within safe ranges - if you are getting NO2- spikes in your fish systems read more into NO3- being turned into NO2- it is vastly over looked in the trade.