Nitrite (NO₂⁻) is the intermediate in nitrification: TAN (NH₃/NH₄⁺) → nitrite (NO₂⁻) → nitrate (NO₃⁻). A healthy biofilter keeps nitrite low. When it rises, conversion has stalled in the middle — common at start-up, after disinfection, when the filter is short of oxygen, or when feed load has risen faster than the bacteria.
Nitrite is toxic even at low levels, especially for salmonids in soft freshwater. It impairs the blood: nitrite is taken up across the gills and converts haemoglobin to methaemoglobin, which carries oxygen poorly ("brown blood"). The fish can therefore suffocate even when DO in the water looks fine. See Ammonia / TAN and dissolved oxygen.
Why nitrite appears
Ammonia-oxidising bacteria (first step) usually establish faster than nitrite-oxidising bacteria (second step). That is why a nitrite spike often comes after TAN has started to fall — the classic "new biofilter" pattern. The same can happen if the filter is knocked out and then recovers in the wrong order.
- Start-up or restart of the biofilter.
- Too fast an increase in feed or biomass.
- Low DO or low alkalinity in the filter.
- Disinfection, medication or a sudden pH drop that hits nitrifiers.
- Dead matter or fouling that creates oxygen-poor zones.
Chloride protection
Chloride (Cl⁻) competes with nitrite at the gill surface. Higher chloride reduces nitrite uptake — that is why salt is often used in freshwater during a nitrite event. It protects the fish temporarily. It does not remove the nitrite and it does not fix the biofilter. The root cause is always that the second step of nitrification is not keeping up.
Salmonids in soft, low-chloride water are among the most sensitive. In brackish water and the sea there is already a lot of chloride, so the same nitrite is usually less dangerous — but a large spike is still a process fault. No dose here: calculate against species, measured nitrite and existing chloride/salinity. See salinity and mineral and ion management.
Measurement: NO₂⁻ or NO₂-N
Kits and labs may report nitrite as NO₂⁻ or as nitrite-nitrogen (NO₂-N). The numbers are not the same: NO₂⁻ ≈ 3.3 × NO₂-N. Only compare values in the same unit. Measure together with TAN and nitrate so you see where the chain is stuck: high TAN = first step lagging; high nitrite and low TAN = second step lagging; both high = the filter is not keeping up at all.
Orientation — not a limit value
The aim in intensive freshwater culture is in practice "as close to zero as the filter can hold". Salmonids and early stages tolerate the least. Warm-water species often tolerate more. Even moderate levels over time cut growth and stress tolerance. Use species, chloride and exposure time — not a generic number from another farm.
Nitrogen chain — levels for rainbow trout
Below are approximate orientation levels for rainbow trout (Oncorhynchus mykiss) in freshwater grow-out. All values are as nitrogen (mg/L N). The diagram uses the same logarithmic scale for every row (0.01–300 mg/L N) so concentration differences are obvious: NH₃ is in thousandths, nitrate in tens to hundreds. NH₃ depends on TAN, pH and temperature (calculator under Ammonia / TAN). Hatchery and fry often need tighter targets. Not permit values.
In short: keep NH₃-N preferably below about 0.012 mg/L (above about 0.05 risk rises clearly). TAN below about 0.5 mg/L is a common soft target when pH is moderate — but NH₃ governs toxicity. NO₂-N in soft freshwater should sit near zero (below about 0.1 mg/L as an operating target; above about 0.5 is clear risk). NO₃-N below about 50 mg/L is often good long-term; 50–100 can be acceptable on many farms; above about 200 plan more exchange, denitrification or nutrient recovery.
| Parameter | Target / low | Elevated | High risk | Too high (long-term) |
|---|---|---|---|---|
| NH₃-N | < 0.012 | 0.012–0.05 | 0.05–0.20 | > 0.20 |
| TAN (som N) | < 0.5 | 0.5–1.5 | 1.5–3 | > 3 |
| NO₂-N | < 0.1 | 0.1–0.5 | 0.5–1.5 | > 1.5 |
| NO₃-N | < 50 | 50–100 | 100–200 | > 200 |
mg/L as nitrogen (N). Rainbow trout, freshwater, grow-out — teaching orientation.
Nitrogen — orientation levels (rainbow trout, freshwater)
mg/L N (same log scale for all)
Same scale for all rows: logarithmic 0.01–300 mg/L as nitrogen (N). That makes the gap between NH₃ (thousandths) and NO₃ (tens–hundreds) obvious. Green = common operating target, yellow = elevated, orange = clear risk, red = unacceptable for long-term operation; grey = outside that parameter's typical band. TAN depends strongly on pH/temp. Orientation for grow-out rainbow trout — not hatchery, not a regulatory limit.
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What you do when nitrite rises
- Protect the fish: cut feed load, keep DO high, consider temporary chloride protection in freshwater (no dose here).
- Fix the filter: DO and alkalinity in the biofilter, remove sludge, stop hitting the bacteria, do not raise feed until nitrite falls.
- Dilute if you have water: more make-up lowers nitrite quickly but does not fix an undersized filter.
- Follow TAN, nitrite and nitrate together until the chain is whole again.
Differences between systems
- RAS: nitrite is a process parameter. Start-up and load increase are the usual risk windows.
- Flow-through: nitrite is less common if the source is clean; it can appear with reuse or a biofilter stage.
- Ponds: algae and sediment affect nitrogen; nitrite can rise when the bottom goes low in oxygen.
- Marine: more chloride gives some protection, but a nitrite spike still means treatment is lagging.
Signs of nitrite stress
Increased gill ventilation, lethargy, gathering at the surface or inlet, darker colour, reduced appetite. Gills and blood can look brown in severe methaemoglobinaemia. The picture resembles low DO, high CO₂ and ammonia — measure NO₂⁻, TAN, DO, pH and temperature before changing everything at once.
Practical rule of thumb
TAN shows whether the first step is keeping up. Nitrite shows whether the second step is keeping up. Nitrate shows what has accumulated over time. Salt can buy time. Only the biofilter removes the problem. See Ammonia / TAN, alkalinity and nitrate.
Sources/overview (selection): Timmons & Ebeling, Recirculating Aquaculture; Boyd & Tucker; established RAS practice on nitrification and chloride protection. Orientation, not permit values. No dose in the text.