Nitrite is the middle child of the nitrogen cycle — less famous than ammonia, less visible than nitrate, and the thing that quietly kills more fish during cycling than either of its siblings. I have lost count of the messages I get from new aquarists who say "ammonia is zero, the tank is cycled, why are my fish gasping?" The answer is almost always nitrite. Ammonia drops to zero in week 3 of a cycle, the fish keeper declares victory, fish go in, and then nitrite — which has been climbing the whole time — peaks in week 4 and kills them.
This is the deep-dive companion to the water parameters overview. It covers the chemistry of nitrite, the biology of the bacteria that produce and consume it, the mechanism by which it kills fish, and the practical steps to survive a nitrite spike. Read this alongside the ammonia guide — the two compounds are part of the same chain, and you cannot understand one without the other. The upcoming nitrate guide completes the trio.
Nitrite should always read 0 ppm. Any visible purple on your API test is a problem. The fish can survive low levels longer than they can survive ammonia, but the damage accumulates — and a high nitrite spike can kill just as fast.
What Is Nitrite?
Nitrite is a negatively charged ion made of one nitrogen atom and two oxygen atoms — chemical formula NO₂⁻. It forms when bacteria oxidize ammonia, stripping hydrogen atoms off the NH₃ molecule and attaching oxygen atoms in their place. The bacteria responsible are Nitrosomonas (and a few related genera like Nitrosococcus and Nitrosospira), and they do this oxidation as their sole energy source — they are chemolithotrophs, getting energy from a chemical reaction the way plants get energy from sunlight.
Nitrite is unstable in an aquarium with a healthy bacterial colony. It exists only as a transient intermediate — produced by Nitrosomonas and immediately consumed by Nitrospira bacteria (older textbooks call them Nitrobacter, but DNA testing on aquarium filters shows Nitrospira does the actual work). In a cycled tank, the nitrite you produce today is gone tomorrow; the Nitrospira colony is large enough to keep up. In an uncycled or crashed tank, nitrite accumulates because there are not enough Nitrospira to eat it.
The toxicity of nitrite is dose-dependent and species-dependent. Concentrations as low as 0.25 ppm begin to affect sensitive species (rainbowfish, some tetras, all shrimp). At 1 ppm, most fish show symptoms. At 5 ppm and above, death follows within hours to days depending on species, temperature, and the fish's prior acclimation. The general rule is the same as for ammonia: zero is the only acceptable number.
The Bacteria That Make Nitrite
Nitrosomonas bacteria are the workhorses of the first half of the nitrogen cycle. They live on every surface in your tank but concentrate in your filter media, where constant water flow delivers fresh ammonia and oxygen. They reproduce by binary fission every 7 to 24 hours under ideal conditions (warm water, neutral pH, plenty of oxygen) — slow compared to most bacteria, which is why cycling takes weeks instead of days. A single Nitrosomonas cell in a new filter will become a colony of a few thousand in two weeks, and a few million in four weeks.
The reason nitrite spikes happen at all is that Nitrosomonas and Nitrospira do not colonize at the same rate. Nitrosomonas grows first, faster, and dominates the early filter. By week 2 or 3 of a cycle, you have enough Nitrosomonas to convert all your ammonia to nitrite — but you do not yet have enough Nitrospira to convert that nitrite to nitrate. The nitrite accumulates. This is the "nitrite spike" that every cycle goes through, and it is the most dangerous period of any new tank.
Nitrospira reproduces more slowly than Nitrosomonas and is more sensitive to low pH (below 6.5, it goes dormant), low oxygen, and certain medications. It typically takes 5 to 7 weeks to fully colonize a new filter, compared to 2 to 3 weeks for Nitrosomonas. This is why a tank can read 0 ammonia at week 3 and still not be safe for fish — the second colony has not caught up. The cycle is not complete until both bacteria are working in sequence, and the second one always lags.
Why Nitrite Is Toxic — Brown Blood Disease
Nitrite kills by the same mechanism as ammonia and carbon monoxide: it blocks oxygen transport in the blood. Fish hemoglobin (the protein in red blood cells that carries oxygen) reacts with nitrite to form methemoglobin, which is brown and cannot bind oxygen. The blood literally turns from red to brown — hence the name "brown blood disease" — and loses its ability to deliver oxygen to tissues. The fish suffocates internally even in fully oxygenated water.
The visible symptoms are distinctive. Fish gasp at the surface (where oxygen is highest), hang near the filter outlet or air stone, breathe rapidly with flared gills, and become lethargic. Their gills may look brownish rather than bright red. In severe cases, the fish loses color, hangs motionless at the bottom, and dies within hours. A tank with active nitrite poisoning often has all the fish crowded near the surface in one corner — they are not schooling, they are suffocating.
The gill damage from nitrite is reversible if caught early — the methemoglobin converts back to hemoglobin over a few days once nitrite is removed. But prolonged exposure causes secondary gill damage from oxygen starvation, and the fish becomes vulnerable to bacterial infections during recovery. Even after the spike clears, fish that survived a serious nitrite event often die in the following weeks from secondary infections. Catching a nitrite spike early is the difference between fully recovered fish and permanently damaged ones.
The chloride ion (Cl⁻) competes with nitrite (NO₂⁻) for uptake through the gills. Adding aquarium salt (sodium chloride) raises the chloride concentration in the water, which physically blocks nitrite from entering the fish's bloodstream. This is one of the few cases in the hobby where a chemical additive has a direct, measurable, biological mechanism — not snake oil.
How to Test for Nitrite
Use a liquid test kit. The API Freshwater Master Test Kit includes a nitrite test that uses the diazotization method: you add five drops of reagent to a vial of tank water, shake, wait five minutes, and compare the color to a chart running from blue (0 ppm) through purple to deep magenta (5+ ppm). The detection floor is 0.25 ppm — any visible purple tint above pure blue means nitrite is present.
Test nitrite daily during the first 6 weeks of any new tank, after any medication course, after a filter cleaning, and whenever fish show any signs of respiratory distress. In an established tank that has been stable for months, weekly nitrite testing is sufficient alongside ammonia and nitrate. If you ever see fish gasping at the surface, test nitrite and ammonia before you do anything else — those two parameters explain 90% of acute respiratory distress in aquarium fish.
Test strips exist for nitrite but are even less reliable than the ammonia strips. The pad chemistry degrades faster than liquid reagents, the color matching is harder (purple is harder to read than yellow-green), and the detection floor is worse (often 0.5 ppm, missing early spikes). Skip the strips; the liquid kit costs the same per test over a year and is dramatically more accurate.
Symptoms of Nitrite Poisoning
The first sign is usually behavioral, not physical. Fish gather near the surface, often in the corner with the strongest water flow from the filter outlet. They breathe rapidly — gills moving visibly faster than normal, sometimes with the operculum (gill cover) flared. They may "yawn" repeatedly, a behavior where they stretch their jaws wide to push more water over the gills. Net them at this stage and their gills will look brownish-red rather than the bright cherry red of healthy gill tissue.
As poisoning progresses, fish become lethargic. They stop eating. They may sink to the bottom and lie on their side, gills barely moving. This is the late stage — death follows within hours. At this point, even an immediate 80% water change may not save them; the methemoglobin takes hours to convert back to hemoglobin, and the fish may die of oxygen starvation during the recovery period. Early intervention is everything.
One thing to watch for: nitrite poisoning can mimic other diseases. Fish gasping at the surface can also be a sign of low oxygen (warm water, no surface agitation), gill flukes, or ammonia poisoning. Test before you treat — adding medication to a tank that actually has a nitrite problem makes everything worse, because most medications further damage the bacterial colony and the nitrite spike gets worse, not better.
How to Fix a Nitrite Spike
The fix has four steps, done in this order. Step 1: water change. A 50% water change with dechlorinated, temperature-matched water immediately halves the nitrite concentration. This is the single most effective intervention. Repeat daily until nitrite reads 0.
Step 2: add aquarium salt. Dose 1 teaspoon of plain sodium chloride (no iodine, no anti-caking agents, no "aquarium salt with additives") per gallon of tank volume, dissolved in tank water and added slowly over an hour. The chloride ions block nitrite uptake through the gills and buy the fish 24 to 48 hours of grace while you address the spike. This is an emergency measure, not a permanent additive — once nitrite reads 0, do water changes to gradually remove the salt over a week.
Step 3: stop feeding. Do not feed for 24 to 48 hours. Less food means less ammonia produced, which means less nitrite produced. The Nitrospira colony needs time to catch up, and starving the system of new ammonia is the fastest way to let them. When you resume feeding, go to half portions once a day.
Step 4: add bottled bacteria. Products like Dr. Tim's One and Only, Tetra SafeStart, and FritzZyme 7 contain live Nitrospira (the good ones do, anyway). Dosing them during a nitrite spike can shorten the spike by 3 to 5 days by jump-starting the colony that is supposed to be eating the nitrite. Dose once, do not redose daily, and turn off UV sterilizers. The bottleneck is always Nitrospira reproduction, and bottled bacteria are the only way to add live Nitrospira directly.
10 gallon tank: 10 tsp salt (~50g). 20 gallon: 20 tsp (~100g). 55 gallon: 55 tsp (~275g). Dissolve in a liter of tank water first, then add slowly — never dump crystals directly into the tank, they can land on fish and burn them. Scale-less fish (loaches, catfish, eels) tolerate salt less well; halve the dose for tanks with these species.
Nitrite's Relationship to Ammonia
Nitrite only exists because ammonia exists. There is no source of nitrite in a typical aquarium other than the bacterial oxidation of ammonia — fish do not excrete nitrite directly, food does not contain nitrite, tap water does not contain nitrite (unlike ammonia, which can come in via chloramine). If you see nitrite, you can be certain ammonia was present in the recent past, even if your current ammonia reading is zero.
This is why a "0 ammonia, high nitrite" reading is so common during cycling — it means your Nitrosomonas colony is working but your Nitrospira colony is not. The ammonia is being produced (by fish or by your dosing), it is being converted to nitrite (by Nitrosomonas), and then it is piling up because nothing is converting it to nitrate. From the fish's perspective, this is actually slightly safer than having ammonia present — nitrite is less immediately toxic than NH₃ — but it is still dangerous and needs to be addressed.
The other implication: if you fix an ammonia spike by killing the Nitrosomonas (for example, by running a UV sterilizer during a medication course), the nitrite level will also drop, but only because no more nitrite is being produced. The underlying Nitrospira colony is fine. The reverse is not true — killing Nitrospira does not affect ammonia levels, but it causes nitrite to spike. When you medicate a tank, assume both colonies are damaged and test for both compounds daily for a week afterward.
Frequently Asked Questions
Is nitrite more dangerous than ammonia?
Slightly less, but in the same league. Both compounds prevent fish blood from carrying oxygen, but ammonia also directly burns gill tissue, which nitrite does not. In practice, nitrite is marginally less lethal per ppm, but the spike lasts longer because Nitrospira (the bacteria that eat nitrite) take 1 to 2 weeks longer to colonize than the Nitrosomonas that eat ammonia. A nitrite spike often stretches across week 3 to week 5 of a fish-in cycle and can persist for days if you do not intervene.
How much aquarium salt should I add for nitrite poisoning?
The standard emergency dose is 1 teaspoon of plain aquarium salt (sodium chloride, no additives) per gallon of tank volume, dissolved in tank water and added slowly over an hour. The chloride ion competes with nitrite for uptake through the gills, effectively blocking nitrite from entering the fish's bloodstream. This is a temporary measure while you do water changes and wait for Nitrospira to colonize; it is not a substitute for fixing the underlying spike. Do not use table salt with iodine or anti-caking agents.
How long does a nitrite spike last?
In a new tank with no seeded media, a nitrite spike typically lasts 1 to 3 weeks — usually peaking around week 3 of a fishless cycle and clearing by week 5 or 6. In an established tank with a damaged cycle, a nitrite spike clears in 5 to 10 days with daily water changes and bottled bacteria. The bottleneck is always Nitrospira reproduction, which doubles every 20 to 24 hours under ideal conditions and slower at low pH or temperature.
Why does my tank have nitrite but no ammonia?
This is the normal middle stage of a cycle. Your Nitrosomonas colony (ammonia-eaters) is working well enough to keep ammonia at zero, but your Nitrospira colony (nitrite-eaters) has not caught up yet. The ammonia is being converted to nitrite faster than the nitrite is being converted to nitrate. This is most common in weeks 3 to 5 of a new cycle, or in an established tank where Nitrospira were selectively killed (often by a medication) while Nitrosomonas survived. Keep doing water changes; the second colony will catch up.