Why Is the Nitrite High in My Aquarium? Causes and Safe Level

The short answer

Nitrite rises when the tank makes more waste than its filter bacteria can convert. In a new tank, that is part of cycling; in an established tank it can follow a filter cleaned under the tap, too many new fish, overfeeding, a dead fish or antibiotics in the water. Aqueon and Seachem put the target at zero.

Practical Fishkeeping, a UK fishkeeping magazine, sums up the cause in one line: high nitrite arises when “there’s too much pollution being produced for the beneficial bacteria to cope with.” So the useful question is what changed: the tank’s age, the last filter clean, the stocking or the feeding.

Where nitrite comes from

Fish waste, uneaten food and dead plant matter break down into ammonia. Aqueon, an aquarium equipment and water-care maker, explains the next two steps: nitrifying bacteria living in the filter, the gravel and other surfaces “convert ammonia to nitrite (NO₂) which is also toxic”, and “a different set of nitrifying bacteria” then turns nitrite into nitrate. Practical Fishkeeping names the bacteria: Nitrosomonas handle the ammonia, and Nitrospira and Nitrobacter handle the nitrite. The magazine calls the nitrate at the end only mildly toxic by comparison.

Aqueon puts the order plainly: “A rise in nitrite usually follows an ammonia spike.” When the second group of bacteria is missing, too small or damaged, nitrite piles up instead of moving on to nitrate. It matters because API, a test-kit maker, says on its nitrite test kit page that “low levels of nitrite inhibit respiration and suppress the immune system; high levels cause suffocation.”

Six reasons nitrite climbs

Practical Fishkeeping’s summary covers most cases: “Overfeeding and overstocking can lead to high nitrite levels, but incorrect filter maintenance and new tank syndrome are perhaps the most common cause.”

The tank is new and still cycling

A new filter has too few bacteria for the waste going in. Aqueon’s nitrogen cycle guide, written for freshwater tanks, says that after fish go into a new tank, “by the second week, you should see nitrite levels rising”, and that the whole cycle “typically takes three to eight weeks”. Clue: the tank is within its first few weeks, and ammonia rose before nitrite did. Our guide on why an aquarium won’t cycle covers the reasons a cycle stalls.

The filter media was cleaned under the tap or replaced

Practical Fishkeeping notes that new filters, and “media which has been washed in tapwater or replaced with new”, hold few bacteria and cannot remove much waste. Clue: nitrite rose within days of a filter clean or a new cartridge. Its rule: “Only wash filter media in old tank water, not under the tap, and always replace media bit by bit, not all at the same time.” Our guide to cleaning a hang-on-back filter with tank water explains the method.

Too many fish went in at once

Aqueon warns that adding too many fish at once to a new tank “will cause ammonia and nitrite to rise to unsafe levels”, a condition called New Tank Syndrome. Practical Fishkeeping lists overstocking as a cause in any tank. Clue: the reading climbed after a big group of new fish went in.

The fish are being overfed

Uneaten food rots into ammonia, then nitrite. Aqueon lists uneaten food as a source of waste, and its feeding rule for new tanks is to “feed only what your fish can consume in 2 minutes or less.” Clue: food left on the substrate or in the plants after feeding.

A fish has died out of sight

Glass Grown Aquatics, an aquarium plant and fish retailer, writes on its blog that dead fish and leftover food produce ammonia, and that a sudden spike in a mature tank often means something is off, “maybe overfeeding or a dead fish hidden in the plants”. Its advice is a head count at feeding time. Clue: someone is missing.

Antibiotics in the water reached the filter

The University of Florida’s IFAS Extension, a university extension service, advises against antibiotic bath treatments in any tank where the treated water touches the biological filter, because “the antibiotics may kill or inhibit the nitrifying bacteria in the biological filters”. Clue: nitrite rose during or after an antibiotic treatment dosed into the main tank. Medication itself is a question for an aquatic veterinarian.

A sudden rise in an established tank is a warning in its own right. Fluval, a test-kit maker, says in its nitrite kit’s instructions that “a sudden spike in the nitrite usually signals a dangerous imbalance in the system.”

What your test results mean

The target is zero. Aqueon says “the only safe nitrite level is zero”, and Seachem, a water-treatment and test-kit maker, says on its test kit page that “nitrite is toxic and should be undetectable in any well established aquarium.” Where sources set a warning line, they set it low:

Practical Fishkeeping: “anything above 0.1 mg/l should be viewed as unacceptable” and a potential cause of stress.
Fluval: “a concentration over 0.3 mg/L” of nitrite “is considered potentially dangerous.” For an established tank reading close to 0.3 mg/L, its leaflet says “you should test again 24 hours later”; above 0.3 mg/L, it says “A dangerous amount of Nitrite is present in the water. Immediately change 20% of the water.”

On test kits, mg/L and ppm are the same unit (Fluval’s kit is labeled “mg/L (PPM)”). One thing to check: some kits report nitrite-nitrogen (NO₂-N) instead of nitrite. Practical Fishkeeping warns that “some results need converting from nitrite-nitrogen”, and Seachem’s conversion is to “simply divide by 3.3” to go from nitrite to nitrite-nitrogen. Worked from that factor, 0.3 mg/L of nitrite is about 0.09 mg/L as nitrite-nitrogen, so compare like with like before you judge a reading.

Nitrite on its own tells you less than nitrite read alongside ammonia and nitrate:

What the kits show	What it usually means	Next move
Ammonia up, nitrite zero, new tank	Cycle has started; nitrite comes next	Test every 2-3 days
Nitrite up, some nitrate, new tank	Second half of the cycle	Keep testing, small water changes
Ammonia and nitrite zero, nitrate present	Tank is cycled (Aqueon)	Weekly tests
Nitrite up in an established tank	Waste outran the filter	Find the cause above; over 0.3 mg/L, Fluval says “Immediately change 20% of the water”, with conditioner in the new water, added slowly (see Bringing it down)
No change after a big water change	Tap water, test method or old reagents	See the next section

The testing intervals are Aqueon’s for a cycling tank and API’s for an established one.

Seachem notes that during cycling nitrite “may exceed 20 mg/L”. That is a reading a cycle can reach, not a safe one: Fluval’s leaflet calls “a concentration over 0.3 mg/L” “potentially dangerous”, and Aqueon says that while a tank with fish cycles, “You’ll likely need to change out about 10 to 25% of the water every few days to ensure ammonia and nitrites don’t reach toxic levels.”

Nitrite that rises after a water change, or will not drop

A water change dilutes nitrite, so a reading that goes up afterward can point to something done at the same time, or to the new water itself.

Your tap water. Nitrite is one of the contaminants in the EPA’s drinking water standards (“Nitrite (measured as Nitrogen)” in its table), which “apply to public water systems”. So tap from a public system can carry some, up to a limit set for drinking water, not for fish; well water is not covered. Test a glass of tap water with the same aquarium kit. For questions about your drinking water, ask your water supplier or health department.
A filter clean on the same day. Rinsing media under the tap or swapping a cartridge during the water change knocks back the bacteria (see the tap-washed filter cause above).
The test itself. When an owner on a FishLore forum thread reported that nitrite stayed high through big water changes, a member suggested checking the source water, following the kit’s directions exactly, and considering that “the test strip or reagent is faulty / expired”, with a store test of a sample to confirm.

How long high nitrite lasts

In a new tank it lasts until the nitrite-eating bacteria catch up. The makers’ timelines overlap:

Fluval: “there is normally a strong rise in nitrite between 10 days and two weeks after start-up”, and a new aquarium is usually fully established in about one month. “In saltwater environments, this time can almost be doubled.”
Practical Fishkeeping: “it generally takes the bacteria 4-6 weeks to build-up, or mature”, depending on temperature and water chemistry.
Aqueon (freshwater tanks): three to eight weeks for the whole cycle. “Once ammonia and nitrite levels return to zero your aquarium has completed its first cycle,” and Aqueon says that is when you can add more fish.

API’s test kit page sets the testing rhythm: every other day when a tank is first set up, and “once a week after the biological filter has been established (in about 4-6 weeks).”

For an established tank the sources give no timeline. Practical Fishkeeping’s advice is to remove the cause, “test daily and be prepared to make further water changes” until the reading falls.

Bringing it down: the water-side basics

Practical Fishkeeping’s first step is to “find out why the level is high” and stop it happening again. Then:

Change part of the water

For any size of change, follow the two cautions Fluval’s leaflet gives with its 20% change: “Use Tap Water Conditioner in the replacement water” and “Add replacement water slowly to avoid fluctuations in other water characteristics.” The amounts depend on the case. While a freshwater tank cycles, Aqueon expects you to “change out about 10 to 25% of the water every few days”. In an established tank reading over 0.3 mg/L, Fluval says to change 20% at once. Practical Fishkeeping’s advice “upon finding nitrite” is to “make a large (50% or more) partial water change”, then test daily and be ready to repeat it.

Feed less

Practical Fishkeeping’s advice for a new tank or filter is not to add too many fish and not to add much food. Aqueon’s rule for new tanks is food the fish finish within two minutes.

Leave the filter media alone

Avoid swapping cartridges or scrubbing media while nitrite is high, since Practical Fishkeeping notes that replaced or tap-washed media holds few bacteria. If the filter clogs, rinse the media in old tank water, not under the tap, and replace media bit by bit, not all at once. While nitrite is high, Glass Grown Aquatics’ blog notes that seasoned owners rinse a hang-on-back cartridge rather than replace it: “simply rinse debris off and slot it back in”.

Add bacteria, not just water

For a new tank or filter, Practical Fishkeeping suggests moving some media from an established filter (Aqueon says “from an established, healthy aquarium”) or adding a bacterial starter culture. Fluval strongly recommends a biological supplement in a new set-up, and Seachem says its Stability product is designed to “rapidly and safely establish the aquarium biofilter”.

Some conditioners are made for this. Seachem says its Prime conditioner “may be used during tank cycling to alleviate ammonia/nitrite toxicity”, and that it contains a binder that renders nitrite non-toxic so the biofilter can remove it. That is Seachem’s claim for Prime, not for every conditioner, and the bacteria still do the removing, so water changes and testing continue.

Ready to buy?

Our roundup of freshwater aquarium test kits compares liquid kits and test strips for freshwater tanks, and our Water Care & Testing section gathers the test kits and conditioners.

If your fish look unwell

Practical Fishkeeping notes that affected fish often gasp or hang at the surface, but “you can’t tell how much is present unless you test the water.” Aqueon adds that rapid breathing and gasping have other causes too.

Warning

If fish are gasping or hanging at the surface, or look unwell, test ammonia, nitrite and nitrate now and contact an aquatic veterinarian. Water changes and water-care products are designed for the water; this article gives no fish treatment or medication advice.

If ammonia is also above zero, our guide to why aquarium ammonia gets high covers the causes of the first step in the chain. Once nitrite is at zero, nitrate is the number to watch; see why nitrate levels get high in an aquarium.

Frequently Asked Questions

Q. Can fish recover from high nitrite?

That depends on the fish, the level and how long they were exposed, which is a question for an aquatic veterinarian. Your part is the water: find the cause, test daily and bring the reading back to zero.

Q. Why is my nitrite high but my ammonia is zero?

In a new tank that fits Aqueon’s timeline: ammonia rises first and nitrite by the second week, so nitrite can still be up after ammonia has dropped. In an established tank it usually means the nitrite-to-nitrate step is behind, for example after a filter clean or a new cartridge.

Q. Can I add fish while nitrite is high?

Aqueon advises waiting until ammonia and nitrite return to zero before adding more fish, because a new filter cannot yet process much waste.

Q. Does a water conditioner remove nitrite?

Not on its own, and only some conditioners claim to act on nitrite: Seachem says Prime “detoxifies ammonia, nitrite, and nitrate”. The filter bacteria and water changes are what take it out.

Sources (13)

Aquarium Tales Team

Written and reviewed by the Aquarium Tales Team. See the About page for how we work.