mg/L
Nitrite
What is it
Nitrite (NO₂⁻) is an intermediate compound in the aquarium nitrogen cycle. Nitrosomonas bacteria produce it while oxidizing ammonia (NH₃), and Nitrobacter and Nitrospira bacteria subsequently convert it into nitrate (NO₃⁻). It appears when ammonia is being converted but the second bacterial step is not keeping up.
In a mature, fully cycled aquarium, nitrite should always be zero: the bacterial colony in the biological filter converts it to nitrate in real time. Its presence indicates a problem in the nitrogen cycle.
Nitrite is extremely toxic even at very low concentrations (0.1 mg/L already causes stress). It is the most common cause of mortality in the first weeks of a new aquarium ("new tank syndrome").
Why it matters
Nitrite binds to haemoglobin in fish blood, forming methaemoglobin, which cannot carry oxygen. Even in well-oxygenated water, fish suffocate because their blood cannot deliver oxygen to the tissues.
At 0.1 mg/L fish show the first signs of stress: rapid breathing and faded colours. At 0.25 mg/L visible gill damage begins. At 0.5 mg/L the risk of mortality is real, especially for sensitive species such as shrimp and small fish.
Nitrite is especially insidious because fish can appear normal for hours before suddenly collapsing. Gill damage is cumulative and can predispose fish to secondary bacterial infections in the following weeks.
Ideal ranges
Interactions with other parameters
High nitrite inhibits nitrifying bacteria themselves, creating a vicious cycle. Nitrite usually follows ammonia spikes and precedes nitrate accumulation; low oxygen, medication, chlorine or filter cleaning can all slow the bacteria that remove it. It also lowers fish tolerance of pH and temperature changes.
| 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 levels (0.1-0.25 mg/L) cause subacute stress, rapid breathing, dull colours and reduced appetite. Shrimp stop feeding and shelter in corners.
Dangerous levels (0.25-1.0 mg/L) cause dark, visibly chocolate-brown gills, surface gasping, erratic swimming and mortality in the most sensitive species. This is an emergency to treat immediately with a water change.
Critical levels (above 1.0 mg/L) make mass mortality within 24-48 hours likely. Even surviving fish can have permanent gill damage and a shortened life expectancy. Make an immediate 70-80% water change and repeat every 12 hours.
Common Myths
- A little nitrite is acceptable in an established tank; the target is zero.
- Clear water means safe water; nitrite is invisible.
Measurement and adjustment
How to measure
A liquid reagent test (API Nitrite, JBL NO₂) is the standard method: the sample ranges from pale pink for traces to dark purple at high levels. Compare it with the supplied colour chart.
In a mature aquarium, the result should always be the palest possible zero. Any colour other than the palest pink needs attention. During cycling, test frequently until both ammonia and nitrite read zero after feeding or dosing ammonia.
Testing nitrite is essential during initial cycling (test every 2-3 days), after adding fish, after medication that can kill filter bacteria, and after any unexplained mortality.
How to adjust
For an immediate emergency, change 50-70% of the water with dechlorinated water at the same temperature. Add aquarium salt (1 g/L of NaCl), which reduces nitrite toxicity by competing for uptake at the gills. Increase aeration.
In the short term, use nitrifying-bacteria products (Seachem Stability, API Quick Start) to accelerate colonisation. Zeolite in the filter absorbs ammonia, reducing the upstream load. Reduce or stop feeding, especially if nitrite appears after adding livestock.
In the long term, confirm filter maturity (a complete cycle takes 4-8 weeks), never clean all filter media at once or rinse biological media under untreated tap water, avoid adding too many fish at once, and avoid overfeeding that produces excess ammonia.
Pro Tips
- If nitrite is > 0.5 ppm, do not rely only on water changes: dose non-iodised coarse salt (1 gram per litre). Chloride competes with nitrite at the gills and helps prevent blood poisoning.
- The nitrite peak during cycling usually arrives in the 2nd-3rd week and lasts longer than the ammonia peak.