NO₂ is transformed from NH₃/NH₄, and is the middle product of protein decomposition by bactearia. Even when NO₂ level is only slightly above 0.1mg/1 , it will poison fish . NO₂ usually causes certain symptoms such as , bad appetite , fading body colors, insufficient oxygen level , vulnerable to fish diseases, Accordingly, NO₂test becomes a very important work . NO₂ level had better be maintained at 0mg/1 in aquariums.