Nitrite (NO2) is among the most toxic substances for fish in aquariums. It can enrich in the water in case of insufficient biological filtration. The sera NO2-Test allows monitoring the nitrite level in fresh and marine water easily and reliably. We recommend using sera bio nitrivec (freshwater), sera marin bio reef clear (marine water) or sera pond bio nitrivec (pond) in case of slightly elevated nitrite levels. In case of severe nitrite problems, sera toxivec (freshwater) or sera pond toxivec (pond) quickly provide help. Sufficient for approx. 75 measurements. Product Dimensions: 1.6 x 3.5 x 5.5 inches ; 1.92 ounces Shipping Weight: 2.9 ounces