Nitrite in the aquarium: why it poisons fish and what to do
Nitrite (NO2) is one of an aquarium's worst enemies, all the more dangerous for being invisible. A reference review showed that nitrite, absorbed across the gills, disrupts multiple physiological functions and turns haemoglobin into methaemoglobin, which cannot carry oxygen (research on Google Scholar). A poisoned fish literally suffocates, even in well-oxygenated water. Find our other pointers in the care and grooming section.
Where nitrite comes from
Nitrite appears at the heart of the nitrogen cycle: bacteria turn ammonia (from waste and food) into nitrite, then other bacteria turn that nitrite into far less toxic nitrate. A nitrite spike signals that the second step is not yet in place or has been disrupted. Our guide to water tests and the nitrogen cycle details this mechanism.
When the spike happens
The most classic case is a poorly cycled new tank stocked too soon: the well-known new-tank syndrome. But a spike can also follow an overly aggressive filter clean, a prolonged filtration stoppage, or a sudden overstocking. We explain why you should never stock an uncycled tank in the nitrogen cycle guide.
Recognising poisoning
Fish breathing fast at the surface, lethargy, loss of appetite, sometimes brownish gills: these signs should alert you and prompt an immediate test. The only reliable way to confirm remains a water test, since nitrite is invisible and odourless at these concentrations.
Reacting fast and well
Facing a spike, do repeated partial water changes to dilute the nitrite, stop feeding temporarily (less waste), and support oxygenation. Adding salt (chloride) reduces nitrite toxicity in many freshwater species. Take care not to confuse this spike with the simple cloudy water of a new tank, a separate phenomenon covered in cloudy water in a new aquarium.
Frequently asked questions
What nitrite level is dangerous?
Ideally, nitrite should be undetectable (0 mg/L). Any measurable value indicates an incomplete or disrupted cycle and a risk to the fish.
Does salt really help against nitrite?
Yes: chloride competes with nitrite at the gills and reduces its uptake, a useful stopgap while the cycle recovers.
How long does a nitrite spike last?
As long as it takes the bacteria to establish, often one to several weeks. Regular water changes protect the fish during that period.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.