Nitrite Test Kit Guide
Why nitrite is the second toxic stage of the nitrogen cycle, how a test kit confirms cycling progress, and why any reading above zero is a warning.
What it measures
A nitrite test kit measures the concentration of nitrite, written NO2-, in aquarium water. Nitrite is the second stage of the nitrogen cycle: beneficial bacteria such as Nitrosomonas species convert ammonia into nitrite. The result is reported in parts per million (ppm).
Why nitrite is dangerous
Nitrite is toxic to fish and, like ammonia, can burn fish gills and skin. It is the second toxic stage of the cycle. Any detectable nitrite in a stocked tank indicates that the biological filter is not yet able to keep up with the bioload, and fish are at risk.
Its place in the nitrogen cycle
In the nitrogen cycle, ammonia is first converted to nitrite by Nitrosomonas-type bacteria. A second group of bacteria then oxidises the nitrite into nitrate, the least toxic of the three compounds. Tracking nitrite shows whether this second bacterial population has established itself. The second group of bacteria typically multiplies after the first, which is why nitrite tends to lag behind ammonia during the maturing of a new tank.
Reading the cycle
During cycling, nitrite typically rises after ammonia begins to fall, then drops back to zero as the second bacteria multiply. A cycle is considered complete when both ammonia and nitrite read 0 ppm while nitrate is present. A persistent nitrite reading means the cycle is not finished.
Target reading
The target in a stocked aquarium is 0 ppm nitrite at all times. Any non-zero result calls for a water change to dilute the nitrite and a review of feeding, stocking, and filtration until the reading returns to zero.
When to test
- Daily while a new tank is cycling, especially after ammonia starts to drop.
- After adding fish or increasing the bioload.
- Whenever fish appear stressed or after disturbing the filter media.
- As part of routine monitoring in an established tank.
Using the kit
- Fill the test tube to the marked line with tank water.
- Add the reagent drops exactly as instructed; cap and shake if required.
- Wait the full development time before reading the colour.
- Match the colour to the chart in neutral light and rinse the tube after use.