mg/L
Nitrite
What is it
Nitrite (NO2) is an intermediate waste product in the nitrogen cycle. It appears when ammonia is being converted but the second bacterial step is not keeping up.
Why it matters
Nitrite blocks oxygen transport in fish blood and can become dangerous even when the water looks clear. In a mature aquarium it should stay at zero.
Interactions with other parameters
Nitrite usually follows ammonia spikes and precedes nitrate accumulation. Low oxygen, medication, chlorine or filter cleaning can all slow the bacteria that remove it.
Ideal ranges
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 0 | 0 | mg/L |
| Planted high-tech | 0 | 0 | mg/L |
| Planted low-tech | 0 | 0 | mg/L |
| Shrimp tank | 0 | 0 | mg/L |
Out of range: what happens
Detectable nitrite can cause gasping, red or brown gills, lethargy and deaths during cycling or filter crashes. Sensitive fish and fry are affected first.
Common Myths
- •A little nitrite is acceptable in an established tank; the target is zero.
- •Clear water means safe water; nitrite is invisible.
How to measure
Use a liquid NO2 test kit. During cycling, test frequently until both ammonia and nitrite read zero after feeding or dosing ammonia.
How to adjust
Stop overfeeding, perform water changes, protect the filter bacteria and add aeration. In freshwater emergencies, a measured chloride source can reduce nitrite uptake while the biofilter recovers.
Pro Tips
Never rinse biological media under untreated tap water during a nitrite spike.
If nitrite appears after adding livestock, reduce feeding until tests return to zero.
