The activity of the bio-filter in the aquarium system can be divided in mineralization and nitrification. The product of the mineralization process are ammonia and organic acids. The next step of decomposition is the oxidation (nitrification) of ammonia and ammonium to Nitrite (NO2). Nitrite has a toxic effect on fish because it oxidizes hemoglobin to methemoglobin. While hemoglobin is responsible for the transport of oxygen, methemoglobin lacks this ability!
Nitrite concentrations higher than 0,05mg/Lt (ppm) of NO2 are not acceptable keeping aquarium fishes.
For these reasons, the periodical analytical valuation of nitrite in the aquarium water should be considered, specially during the aquarium set-up and, periodically, to avoid the activity loss of the bio-filter system.