What your nitrite reading means
Good0 ppm
The bacteria that turn nitrite into nitrate are keeping up.
Act todayAny reading above 0
Change 30 to 50 percent of the water today and every day it reads above 0; feed lightly.
For a tank with fish. The same bands run in the test reading checker, which reads all your results together.
Where it comes from
Nitrite is the middle step of the nitrogen cycle. One group of bacteria turns fish waste ammonia into nitrite; a second group turns nitrite into nitrate. In a new tank the second group grows later than the first, so nitrite rises after ammonia and falls last. In an established tank, nitrite means the second group has been set back: a filter cleaned in tap water or replaced all at once, a power cut, some medications, or a sudden jump in feeding or fish.
Why it harms fish
Nitrite passes into the blood through the gills and stops it carrying oxygen, so fish gasp at the surface even in well-aerated water. The University of Florida puts the harmful level as low as 0.1 ppm, which is why any reading counts with fish in the tank.
Bringing it down
- Change 30 to 50 percent of the water with dechlorinated water at the same temperature, today and every day it reads above 0.
- If the pH is below about 6.5 and ammonia is present too, change a little every day instead; a big change can turn the ammonia toxic.
- Feed lightly, keep the filter running and leave the filter media alone.
- Add no new fish until ammonia and nitrite have read 0 for a week.
Keep the bacteria working
Nitrifying bacteria need oxygen and carbonate (KH) to work. Below about 20 ppm of alkalinity they stop working well, according to the University of Florida, so nitrite and ammonia rise in very soft water or an old tank whose KH has run out. KH and pH crashes