Nitrite in Fish Tanks: Safe Levels & Toxicity Table

Nitrite

Written by The Aquarium Calculators Editorial Team Updated on 2026-05-22

NO2- • Type: Inorganic nitrogen ion, second stage of the nitrogen cycle • Aliases: NO2, NO2-, Brown Blood Disease (associated condition)

See safe vs. dangerous nitrite levels by pH, what nitrite does to fish, and the fastest way to bring a spike down.

Molecular Dynamics & Toxicity Thresholds

⚠️ Pathology & Physiological Impact

Nitrite is absorbed through the gills into the bloodstream, where it oxidizes hemoglobin into methemoglobin, which cannot carry oxygen. This condition is commonly called 'brown blood disease' because affected fish's blood and gills take on a brown tint. Fish effectively suffocate even in well-oxygenated water.

Equilibrium Modifiers & Influences:

⚡ Chloride level ⚡ Bioload ⚡ Filtration capacity ⚡ Salinity

Reaction Network & Bio-Chemical Synergy

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Ammonia relatedTo

Produced when Nitrosomonas bacteria oxidize ammonia.

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Nitrate relatedTo

Oxidized by Nitrospira/Nitrobacter bacteria into nitrate.

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Chloride affectedBy

Chloride ions competitively block nitrite uptake at the gill, which is the basis of salt-based nitrite treatment.

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Salt Dosing Calculator uses

What Nitrite Is

Nitrite (NO2-) is the second stage of the aquarium nitrogen cycle, produced when Nitrosomonas bacteria oxidize ammonia. Like ammonia, it has no safe level above zero in a healthy tank — any positive reading means the second stage of your biological filter hasn't yet caught up with the first.

How Nitrite Poisons Fish

Unlike ammonia, which burns gill tissue directly, nitrite works by entering the bloodstream through the gills and converting hemoglobin into methemoglobin, a form that can no longer carry oxygen. This causes fish to effectively suffocate from the inside even in water with normal oxygen levels. The condition is nicknamed "brown blood disease" because of the brownish discoloration it causes in blood and gill tissue.

Recognizing a Nitrite Problem

Affected fish often show rapid gill movement, gasping at the surface, lethargy, and a darkened or brownish tint to the gills — symptoms that can look similar to an ammonia spike but stem from a different mechanism.

Why Nitrite Often Spikes Worse Than Ammonia

During cycling, the bacteria that produce nitrite (Nitrosomonas) typically establish faster than the bacteria that consume it (Nitrospira, and historically described as Nitrobacter). This mismatch creates the well-known "nitrite hump" — a spike that rises higher and lingers longer than the initial ammonia spike before finally dropping to zero once the second bacterial population matures.

Responding to a Nitrite Spike

A large water change is the fastest way to dilute nitrite directly. Aquarium salt (sodium chloride) is a widely used emergency treatment in freshwater tanks, since chloride ions competitively block nitrite from being absorbed at the gill. A detoxifying conditioner such as Seachem Prime also temporarily neutralizes nitrite's toxicity while the biological filter recovers.

Freshwater vs. Marine/Brackish Tolerance

Because marine and brackish systems already carry high chloride/salinity levels, fish in these environments are naturally more resistant to nitrite toxicity than freshwater fish at an equivalent concentration — though a reading of 0 ppm remains the target in any system.

Ammonia vs. Nitrite: Key Differences

← Scroll horizontally to inspect full matrix →

Property	Ammonia (NH3)	Nitrite (NO2-)
Cycle stage	First (produced directly by fish/waste)	Second (produced by bacteria from ammonia)
Toxic mechanism	Burns gill tissue, blocks oxygen absorption	Enters bloodstream, blocks oxygen transport in hemoglobin
Typical spike behavior	Rises first, usually shorter duration	Often spikes higher and lasts longer ('nitrite hump')
Emergency treatment	Water change + conditioner	Water change + conditioner + aquarium salt (freshwater)

Frequently Asked Questions

What does nitrite do to fish, and what is 'brown blood disease'?

Nitrite crosses the gill membrane into the bloodstream and converts hemoglobin into methemoglobin, a form that cannot carry oxygen. Fish suffocate internally even though the water itself has plenty of dissolved oxygen. The nickname 'brown blood disease' comes from the brownish tint this gives to blood and gill tissue. Symptoms include rapid gill movement, gasping at the surface, lethargy, and a darkened or brownish gill color.

How do I lower nitrite quickly in an emergency?

Perform an immediate large water change (50% or more) to dilute the nitrite concentration, and dose a conditioner such as Seachem Prime, which temporarily converts nitrite into a less harmful form. Adding aquarium salt (sodium chloride) is also a well-established emergency treatment for freshwater tanks, since the chloride ions competitively block nitrite from being absorbed through the gills.

Why does nitrite often spike higher and last longer than ammonia during cycling?

The bacteria that consume nitrite (Nitrospira/Nitrobacter) are slower to establish than the bacteria that produce it (Nitrosomonas), so nitrite tends to accumulate faster than it's broken down mid-cycle. This creates the classic 'nitrite hump' in a new tank's cycling graph, where nitrite spikes higher and persists longer than the initial ammonia spike before finally crashing to zero as the second bacterial colony catches up.

Does aquarium salt really help with nitrite poisoning?

Yes. Chloride ions (from sodium chloride, i.e. aquarium or non-iodized table salt) compete with nitrite ions for the same gill uptake channels, meaningfully reducing how much nitrite a fish actually absorbs even while the total nitrite reading stays the same. This is why salt is a standard emergency measure for freshwater nitrite spikes, though it should be used cautiously with salt-sensitive species like scaleless fish, and is unnecessary in tanks that are already at some salinity, like brackish or marine setups.

Why don't saltwater and brackish tanks have the same nitrite problems as freshwater tanks?

Because the existing chloride/salinity in marine and brackish water already occupies the gill uptake pathway that nitrite would otherwise use, fish in these systems are considerably more resistant to nitrite toxicity than freshwater fish at the same nitrite concentration. That said, 0 ppm nitrite is still the target in any system, since very high concentrations can still overwhelm this protective effect.