Nitrites (NO2) in an aquarium: the emergency protocol to lower the level within 48 hours

The test has turned bright pink, or even red, and several fish are staying at the surface with their mouths open. This is not the time to read a chemistry lesson: you need to act, in the right order. To quickly lower nitrites (NO2) in an aquarium, change a large portion of the water with conditioned water at the same temperature, stop feeding, oxygenate the water, and leave the filter alone, then repeat the process the next day until the test reading falls. This protocol does not address the underlying cause of the spike, but it saves the fish while the filter bacteria regain control. Here is how to carry it out over 48 hours.

Why nitrites are an emergency

In an aquarium, fish waste and decomposing food produce ammonia. Bacteria in the filter transform it into nitrites, then other bacteria transform the nitrites into nitrates, which are much less toxic. When the second step does not keep up, nitrites accumulate.

The problem is physiological. Nitrites pass through the gills and transform the hemoglobin in the blood into methemoglobin, a form incapable of transporting oxygen. The fish suffocates in water that is nevertheless well oxygenated: this is “brown blood disease,” described in fish pathology manuals. In an affected fish, the gills take on a brownish tint instead of their usual bright red.

Typical warning signs:

• fish that are breathing rapidly, staying at the surface, or lingering near the filter outlet;
• lethargic fish, resting on the bottom, with folded fins;
• sudden loss of appetite;
• deaths with no external signs of illness, often a few weeks after setup or after cleaning.

Read the test result

In a balanced aquarium, nitrites are undetectable. The thresholds below are commonly used guidelines in freshwater aquariums; sensitivity varies according to species, temperature, and water hardness.

Hour 0: confirm and secure

1. Retest with a liquid drop test if you used a test strip, and check the expiration date. Also measure ammonia if you can: an associated ammonia spike changes the priorities.
2. Remove anything that is rotting: dead fish, food that has fallen behind the decor, melting leaves. A single large dead fish in a small tank is enough to restart massive ammonia production.
3. Stop feeding. Every pellet you distribute will become ammonia, then nitrites. A healthy adult fish can easily go without food for two days.
4. Increase oxygenation: connect an air pump, direct the filter outlet to agitate the surface, and slightly lower the heater if the temperature is above the range required by your species (warm water contains less oxygen).

Hour 1: the major water change

This is the action that really lowers nitrites, immediately and proportionally: replacing 50% of the water halves the level. You can replace anywhere from 30% (moderate spike) to 50% (severe spike), provided you follow three rules.

• Same temperature: a difference of more than 1 to 2 °C adds stress that fish do not need. Measure with a thermometer, not by hand.
• Conditioned water: chlorine and chloramines in tap water kill the filter bacteria. Treat new water with a conditioner before it enters the tank.
• Similar parameters: if your aquarium has very soft, acidic water (reverse osmosis water, filtered rainwater), use the same recipe for the new water. A sudden change in pH or hardness is dangerous, especially if ammonia is present.

Siphon the substrate at the same time in areas where waste accumulates, without turning over all the substrate. Our guide to changing aquarium water explains the technique step by step.

What you absolutely must not do

The most common, and most destructive, reaction is to dismantle and wash the filter with tap water because “the water is polluted.” The nitrites are not in the filter: they are in the water. The filter, on the other hand, is precisely where the bacteria that eliminate them live. Washing it with tap water, changing all the sponges, or replacing the filter media in the middle of a crisis is like removing the firefighters from the fire.

• Do not change 90% of the water at once: you risk parameter shock and do not help the bacteria adapt.
• Do not add new fish, even “to compensate” for losses.
• Do not use medication without a diagnosis: many antibacterial treatments also affect the filter bacteria and prolong the problem.
• Do not turn off the filter at night because of the noise: without water circulation, the bacteria lack oxygen and die within a few hours.
• Do not overdose on “miracle” products that promise to make nitrites disappear in an hour: nothing replaces a water change.

Hours 2 to 24: help the filter regain control

Once the immediate emergency has been dealt with, the goal is to strengthen the population of nitrifying bacteria. Several simple measures:

• Add commercially available live bacteria directly into the filter or near its intake, following the manufacturer's dosage. They do not replace water changes but shorten the critical period.
• Keep the filter running continuously, at a normal flow rate.
• Maintain oxygenated water: nitrifying bacteria consume a lot of oxygen.
• Check the pH and KH: in very acidic water or water with almost no carbonate hardness, nitrification slows significantly. If the KH is very low, raising it gradually helps the filter.

In the evening, test the nitrites again. If they have dropped below half the starting value and the fish are breathing better, you are on the right track.

Hours 24 to 48: the second water change

The next morning, test again. The level often rises slightly overnight because ammonia production continues. This is normal. Depending on the reading:

Salt: an aid, not a universal solution

In fish farming, it is known that chloride ions compete with nitrites at the gills and limit their absorption. Some aquarists therefore add a little non-iodized salt in the event of a spike. This technique has its limitations: many freshwater species (catfish such as corydoras, some characins, shrimp) and many plants do not tolerate it well, and salt only disappears through water changes. If you are considering this option, ask for advice and specify which species are present; in most planted community aquariums, water changes are sufficient.

Special cases

Small aquarium or nano tank

In less than 30 liters, everything happens faster: one extra pellet or one dead snail is enough to send nitrite levels soaring. Water changes need to be more frequent but gentler, for example 30% twice a day rather than 60% at once, to avoid changing the temperature and pH.

Aquarium with shrimp or snails

Freshwater shrimp are highly sensitive to sudden parameter changes. Prepare new water in advance, add it slowly (by drip or in small volumes), and avoid any copper-based product.

Quarantine or hospital tank

A tank set up for the occasion does not have a mature filter: nitrites almost always appear in it. Plan daily water changes and, if possible, place in its filter a sponge that has spent a few weeks in the main aquarium's filter.

After the crisis: monitor for a week

When the test returns to zero, do not let your guard down too quickly. Test once a day for a week, then twice a week. Resume feeding very gradually: a small pinch per day at first, which the fish consume within one minute. If nitrites reappear each time you resume feeding, the filter is not yet sized for your tank's bioload: reduce the population or strengthen the filtration.

Nitrites then turn into nitrates. This is not a short-term problem, but they accumulate: continue regular water changes, around 20 to 30% per week depending on your aquarium. Our article on how often to change the water helps you find the right schedule.

Why the spike occurred

The emergency protocol treats the symptom. Once the fish are out of danger, take the time to understand the cause. The most common causes are:

• an aquarium that has been set up for less than two months, whose nitrogen cycle was not complete when the fish were introduced;
• thoroughly cleaning the filter with tap water, or replacing all the filter media at the same time;
• the introduction of many fish at once;
• prolonged overfeeding, or a vacation with an overly generous automatic feeder;
• a medication treatment that weakened the bacteria;
• a power outage or a filter that has been stopped for several hours.

To clearly distinguish between the different nitrogen compounds and know which one to monitor and when, our guide to essential freshwater tests explains everything.

Also read

To understand where nitrites come from: What are nitrites in an aquarium?.

READ THE ARTICLE

Frequently asked questions

— How can you quickly lower nitrites in an aquarium?

Change 30 to 50% of the water with conditioned water at the same temperature, stop feeding, increase aeration, and do not clean the filter. Repeat 24 hours later if the test remains high.

— What nitrite level is dangerous for fish?

In a balanced aquarium, nitrites should be undetectable. From a few tenths of a mg/L, action is required, and above approximately 0.5 mg/L, the situation becomes an emergency for most species.

— Should you clean the filter when nitrites are high?

No. The filter contains the bacteria that eliminate nitrites. Cleaning it under the tap or replacing its filter media prolongs the crisis.

— How long does it take for nitrites to drop?

With daily water changes and a preserved filter, the level generally drops within a few days. In an aquarium still cycling, it may take one to two weeks for a lasting return to zero.

In summary

• Nitrites prevent fish blood from carrying oxygen: this is an emergency.
• Immediate action: change 30 to 50% of the water with conditioned water at the same temperature.
• Fast for 24 to 48 hours, increase aeration, and remove anything that is decomposing.
• Never wash the filter under the tap or replace all its media during a crisis.
• Live bacteria, tests morning and evening, then investigate the cause once the danger has passed.

Last updated: October 3, 2026.