High nitrogen cycletoxicwater parametertestingnew tank syndrome

Nitrite

By Editorial Team · ·Reviewed by The Aquarium Calculators Editorial Team

NO2- • Inorganic nitrogen ion, second stage of the nitrogen cycle

Nitrite (NO2-) is the second, often more persistent, toxic stage of the aquarium nitrogen cycle, causing 'brown blood disease' by blocking oxygen transport in fish.

Aquarium water test kit showing a nitrite test result

Quick Answer

Nitrite (NO2-) is the second toxic stage of the nitrogen cycle, produced from ammonia and converted into nitrate. It poisons fish by blocking oxygen transport in the blood ('brown blood disease') and should always read 0 ppm; a spike calls for a large water change and, in freshwater tanks, aquarium salt to block nitrite uptake.

Safety Profile

Safe Range
0 ppm-0 ppm ppm
Toxicity Level
High
Toxicity Note
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.

Properties & Reactions

Detection Method Liquid test kits or test strips measuring nitrite (NO2-) directly.
Common Sources Bacterial oxidation of ammonia (Nitrosomonas), New/uncycled tanks, Overstocking during fishless or fish-in cycling, Filter media disruption after ammonia has already converted
Converts To Nitrate (chem-nitrate), oxidized by Nitrospira/Nitrobacter bacteria as the second stage of the nitrogen cycle
Influenced By Chloride level, Bioload, Filtration capacity, Salinity

Related Compounds

chemistry:ammonia

Produced when Nitrosomonas bacteria oxidize ammonia.

chem-nitrate

Oxidized by Nitrospira/Nitrobacter bacteria into nitrate.

chem-chloride

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

Ammonia vs. Nitrite: Key Differences

PropertyAmmonia (NH3)Nitrite (NO2-)
Cycle stageFirst (produced directly by fish/waste)Second (produced by bacteria from ammonia)
Toxic mechanismBurns gill tissue, blocks oxygen absorptionEnters bloodstream, blocks oxygen transport in hemoglobin
Typical spike behaviorRises first, usually shorter durationOften spikes higher and lasts longer ('nitrite hump')
Emergency treatmentWater change + conditionerWater change + conditioner + aquarium salt (freshwater)
Troubleshooting Guide
1 Nitrite spike mid-cycle (the 'nitrite hump')

Expected during cycling. Continue daily water changes and dose a conditioner. Adding aquarium salt can protect fish-in cycles until the second bacterial colony (Nitrospira) matures.

2 Nitrite spike in an established tank

Indicates a filter crash or sudden bioload increase, similar causes to an ammonia spike. Perform a large water change, dose a conditioner, and investigate the underlying cause (overstocking, dead livestock, media cleaned in tap water, etc.).

3 Nitrite persists despite water changes

Check that the filter media hasn't been damaged or replaced entirely, and confirm bioload hasn't been increased faster than the bacterial colony can process. Continued daily water changes plus salt (freshwater only) manage risk while the colony re-establishes.

Continue Learning

Recommended Tools

Nitrite Toxicity Risk Calculator calculatorReference Aquarium Nitrogen Cycle Tracker calculatorReference

Similar Species

Understanding Ammonia in Aquariums: Toxicity & Removal semanticSimilarity

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.
Glossary of Terms
Nitrite (NO2-)
Brown blood disease
Methemoglobin
Nitrogen cycle
Nitrospira
Nitrite hump

Related Content

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Ammonia

Ammonia (NH3/NH4+) is the most toxic compound in the aquarium nitrogen cycle, produced by fish waste and decaying organic matter, and safe only at a reading of 0 ppm.

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pH measures how acidic or alkaline aquarium water is on a 0-14 scale; stability and its effect on ammonia toxicity matter more than hitting one 'ideal' number.