Nitrite poisoning is what happens when a tank makes nitrite faster than its biofilter can clear it. Fish start gasping at the surface even though the water has plenty of oxygen. Their gills turn from red to tan or brown. Fish farmers call it brown blood disease, and they have a cheap, well-tested way to blunt it while the real problem gets fixed: ordinary salt.
This guide explains why nitrite builds up, what it does to a fish, how to read a nitrite test, and what to do in the first hour when the test shows nitrite.
Where nitrite comes from
Nitrite is the middle step of the nitrogen cycle. Fish excrete ammonia, mostly through their gills. Uneaten food and other waste break down into more ammonia. Microbes in the filter turn that ammonia into nitrite, and other microbes turn the nitrite into nitrate, which is far less toxic (SRAC 462).
So nitrite only shows up when the second step falls behind the first. Rotting food doesn't make nitrite directly. It makes ammonia, and the nitrite follows.
The microbe names in older articles are out of date. The nitrite step in freshwater aquariums is done mainly by Nitrospira-like bacteria, not the long-assumed Nitrobacter (Hovanec et al. 1998). Some Nitrospira can even do both steps on their own, a process called comammox (Daims et al. 2015), and a 2024 survey found comammox Nitrospira in all 38 freshwater aquarium biofilters it tested, from both homes and stores (McKnight and Neufeld 2024). The nitrogen cycle guide covers the details. For this article, the practical point is simple: nitrite means the biofilter is behind.
Common causes of a nitrite spike
- A new tank that hasn't finished cycling, or a cycled tank that suddenly got a lot more fish.
- Overfeeding, or a dead fish or snail rotting out of sight.
- Filter damage. Biological media rinsed under tap water, replaced all at once, or left sitting without water flow during a power outage.
- Some medications. Some antibiotics can stall the biofilter. In one study, a single dose of one antibiotic stopped nitrification for two weeks, while several others had no measurable effect (Collins et al. 1976). Test more often during and after any treatment.
Each pound of ammonia nitrogen can turn into about three pounds of nitrite in a short time, so a high ammonia reading is an early warning that nitrite is coming within days (SRAC 462).
Nitrite poisoning: what it does to fish
Nitrite crosses the gills into the bloodstream. There it combines with hemoglobin to form methemoglobin, which can't carry oxygen. The blood turns chocolate brown. The fish can suffocate even when the water is well oxygenated, which is why affected fish gasp at the surface while the air pump is running (SRAC 462; SRAC 451).
Signs of nitrite poisoning:
- Gasping at the surface or hanging near the filter outlet
- Rapid gill movement
- Gills that look tan or brown instead of bright red
- Lethargy and loss of appetite
None of these signs is unique to nitrite. Low oxygen, ammonia and gill parasites can look similar. That's why the test kit comes first.
Species differ a lot. In pond research, largemouth bass and sunfish resisted high nitrite, catfish and tilapia were fairly sensitive, and trout reacted to very small amounts (SRAC 462). Goldfish fell in between catfish and bass. Few tropical aquarium species have been tested, so treat every fish as sensitive.
Fish that survive a nitrite event aren't out of the woods. Catfish farmers see bacterial infections such as Aeromonas and columnaris one to three weeks later (SRAC 462). Watch the tank closely for a few weeks after a spike.
Reading a nitrite test
In a cycled tank, nitrite should read zero. A University of Florida fact sheet on moving ornamental fish lists 0 ppm nitrite as good and anything above 0.5 ppm as bad, and notes that tolerance is species-dependent (UF/IFAS FA-119). Another UF/IFAS fact sheet warns that nitrite can be toxic at levels as low as 0.1 mg/L (FA-16), and the Merck Veterinary Manual uses 0.1 mg/L as its toxicity threshold. Any reading above zero tells you the biofilter isn't keeping up.
Check which unit your kit uses, because kits don't agree. The most widely sold hobby kits read nitrite (NO₂⁻), and one major maker's instructions warn that kits reading nitrite-nitrogen (NO₂-N) give numbers 3.3 times lower. Many lab, aquaculture and veterinary tests report NO₂-N, which is also what most research papers use, and the Merck Veterinary Manual goes as far as saying most test kits do. Your kit's instructions or color card will say which. Multiply NO₂-N by about 3.3 to get NO₂⁻. It's the same trap as with nitrate, explained in the nitrate guide.
Why salt helps: the chloride fix
The chloride in ordinary salt (sodium chloride) competes with nitrite at the gills. With enough chloride in the water, much less nitrite gets into the blood. Fish farmers have relied on this for decades. Keeping at least 10 parts chloride for every 1 part nitrite effectively prevents nitrite from entering catfish, and many catfish farmers hold chloride at 100 ppm year-round as insurance. In ponds, the brown blood clears in about 24 hours after salt goes in (SRAC 462).
Salt doesn't remove nitrite. It buys time while water changes and the biofilter do the real work.
How much salt is that?
These doses are small. The example below works out to about 0.03 g of salt per liter, compared with the 1 to 3 g per liter used as residual salt in many aquariums.
- About 0.62 grams of sodium chloride in 100 US gallons of water raises chloride by 1 ppm. That's about 1.65 mg of salt per liter of water for each 1 ppm of chloride.
- For a 10:1 ratio: chloride to add = (10 × nitrite reading) − chloride already in your water.
- Example: your kit reads 2 ppm nitrite (as NO₂⁻) and your water has no chloride. You need 20 ppm chloride. In 100 actual gallons, that's about 12.5 grams of salt.
A few practical notes:
- Use your tank's true volume, not the number on the box. A tank sold as a 120 often holds closer to 100 gallons once you account for glass, substrate and decor. The tank volume guide shows how to work it out.
- Use the nitrite (NO₂⁻) number in the formula. The pond sources aren't consistent about units. SRAC 462 just says "nitrite." SRAC 451 states the 10:1 ratio as chloride to nitrite-nitrogen (NO₂-N), and aquaculture scientist Claude Boyd writes that chloride at 20 times the NO₂-N concentration completely counteracts nitrite toxicity in channel catfish and probably most other freshwater species. An NO₂⁻ reading is about 3.3 times the same water's NO₂-N reading, so 10:1 on the NO₂⁻ number works out to about 33:1 chloride to NO₂-N, above both of those benchmarks. If your kit reports NO₂-N, multiply the reading by 3.3 before you use the formula.
- Your tap water may already have chloride. Your utility's annual water quality report often lists it. If it's already well above ten times your nitrite reading, extra salt adds little. The tap water guide explains how to read the report. RO water has almost none, so soft-water tanks get little protection from their water.
- Weigh it if you can. As a rough guide, 12.5 grams is about 2 level teaspoons of fine table salt (USDA lists 6 grams per teaspoon), but coarse and flaked salts weigh noticeably less per spoon. Dissolve it in tank water first, and add it slowly. Some keepers with RO or very soft water weigh out one dose ahead of time and keep it, labeled, with the test kit (hobby practice). Keepers generally use plain salt with no added iodine or anti-caking agents (hobby practice).
- Salt only leaves with water changes. Filters don't remove it. Each water change dilutes it, so recheck the ratio while nitrite is still above zero.
Some keepers of soft-water fish and planted tanks prefer not to keep salt in all the time. Using it only during a spike and letting water changes remove it afterward is a reasonable middle ground (hobby practice).
What to do right now: nitrite emergency steps
- Test ammonia and nitrite. Confirm what you're dealing with.
- Change water. A 50% change cuts nitrite roughly in half. If it's still high, repeat over the next day rather than doing one huge change, and match temperature and dechlorinate every time.
- Add salt to reach at least 10:1 chloride to nitrite, using the steps above.
- Stop or cut feeding for a day or two. Less food means less ammonia, which means less new nitrite.
- Add aeration. It won't fix the blood problem, but it helps fish whose blood is carrying less oxygen.
- Find the cause. Look for a dead fish, uneaten food, or a filter that was cleaned too hard or switched off.
- Leave the biofilter alone. Don't replace media or scrub it clean. Rinse mechanical sponges only in old tank water.
- Test daily until nitrite reads zero for several days in a row, then watch for infections over the following weeks.
Nitrite and new fish
The best time to deal with nitrite is before new fish arrive. Test ammonia and nitrite a few days before a shipment. If either reads above zero, sort it out first. A large group of new fish adds a sudden waste load, so a tank that was barely keeping up can spike a few days later.
When the box arrives, float the sealed bag until it matches the tank's temperature, usually 15 to 30 minutes, add an ammonia binder as soon as you open it, and move the fish over with a measured flow of tank water for about 2 to 3 hours; the shipping article has the full steps. After that, test ammonia and nitrite daily for the first week. See the ammonia guide for why ammonia usually moves first.
FAQ
What level of nitrite is toxic to fish?
There's no single safe number, because species differ. In a cycled tank nitrite should read zero. UF/IFAS fact sheets call anything above 0.5 ppm bad and warn that toxicity can start as low as 0.1 mg/L, depending on the species and the chloride in your water. Low chloride, warm water, low oxygen and sick fish all make nitrite more dangerous.
Does a water conditioner remove nitrite?
No. Some conditioners claim to detoxify nitrite for a short time, but nothing in a bottle removes it. Water changes, chloride and a working biofilter are what fix it.
How long does a nitrite spike last?
It depends on the cause. A cycling tank can show nitrite for weeks. A cycled tank that got overloaded often recovers in days once the cause is removed and feeding is cut back. Keep testing daily until it reads zero.
Can fish recover from nitrite poisoning?
Often, yes, if nitrite is brought down and chloride is raised quickly. In ponds, brown blood clears in about a day after salt is added. Survivors are more prone to bacterial infections for a few weeks, so keep the water clean and watch them closely.
Is aquarium salt safe for all fish?
The doses used against nitrite are tiny. In the example above, 20 ppm chloride takes about 33 mg of salt per liter (0.033 g/L), and even 5 ppm nitrite only calls for about 0.08 g/L. Residual salt levels in aquariums are often 1 to 3 g/L, 12 to 90 times more. The usual cautions about salt and scaleless fish, corydoras, live plants and snails are about salt at that treatment strength (hobby practice). The Merck Veterinary Manual notes that some freshwater fish, including wild-caught Amazonian species, elephantnose and related mormyrids, may not tolerate permanent exposure even at 1 to 3 g/L. In one controlled study, bronze corydoras tolerated 1 g/L for 13 weeks (IAAAM 1995). If you keep sensitive species, use the minimum dose for a 10:1 ratio and remove it with water changes once nitrite is gone.
Cover photo (cropped): Britta Beck / Wikimedia Commons, CC BY 4.0
