Aerobic bacteria of the genus Nitrosommonas in the biological filter, oxidize toxic Ammonia to Nitrite. This substance is only slightly less toxic than Ammonia (NH3), but in the biological filter of a well established aquarium Nitrite (NO2-) is quickly transformed into the relatively harmless Nitrate (NO3 -), by yet another aerobic bacteria, called Nitrobacter.
In a newly set up aquarium a high Nitrite level is a normal phenomenon. It means that the Nitrosomonas bacteria have started the nitrification process and produce Nitrite out of Ammonia. Nitrobacter is however not yet present in sufficient quantities to transform Nitrite into Nitrate.
In a newly set up aquarium we advise that you test for Nitrite daily for at least two weeks.
Only this, together with testing for Ammonia (use Red Sea`s Ammonia Test Lab), will tell you when it is safe to put (more) fish into your aquarium. After this two week period, especially in the marine aquarium, one should start testing for Nitrate, using Red Sea`s Nitrate Mini-Lab.
Nitrite should be tested when there is suspicion of a malfunctioning biological filter. After a long period of time, the filter can get dirty causing Oxygen shortage leading to high Nitrite levels. Since in this case toxic Ammonia can also be present in the aquarium, it is advisable to test for Ammonia, using Red Sea`s Ammonia Test Lab.