Nitrite is a toxic nitrogen compound that occurs when there is a disrupted bacterial balance or during the establishment of a new system. The test accurately indicates the nitrite content of the water. Laboratory comparator system to compensate for water coloration: Fill glass cuvettes with sample water, add reagents to one cuvette, place cuvettes in holders, and read values on the color chart. Application: For new freshwater setups: daily for 3 weeks. For new saltwater setups: once a week. In cases of fish diseases or fish deaths.