How to Avoid & Treat Nitrite Poisoning in Your Fish Tank?

Last Updated on February 2, 2019

Nitrite is even more toxic to fish as ammonia and can kill fish. Theoretically, your system will suffer nitrite poisoning if it goes over 6 ppm.

Nitrite posioning
Nitrite Poisoning is caused by adding more fish before nitrifiers grow enough to match the level of ammonia being produced

The nitrites level in the fish tank rises when more pollution is produced for beneficial bacteria to cope with.

Nitrite Spike in a New System

New systems are more prone to the shock of a spike in ammonia especially if the beneficial bacteria have not had a chance to establish themselves.

When nitrifying bacteria develop in an aquaponics system for starting the cycle. the fastest breeding and feeding bacteria are Nitrosomonas that eat the ammonia and produce nitrites. NH3 -> NO2

The other one is nitrospira that convert the nitrites to nitrates (NO2 -> NO3). It has slower reproduction rates and takes more time to establish. Nitrate is the good stuff that plants like to eat and does not have a harmful effect on fish.

Nitrogen Cycle: Ammonia > Nitrite > Nitrate
Nitrogen Cycle: Ammonia > Nitrite > Nitrate

It can result in a “nitrite spike” where you may have high levels of nitrites for a week to three before they start coming down when they are getting converted to nitrates.

Risks of Nitrite Poisoning in Established System

Nitrite poisoning is not just limited to just new systems. Overfeeding and overstocking can lead to high nitrite levels. However, improper filter maintenance and new tank syndrome are perhaps the most common cause. Another reason could be the washing the grow bed because it also washes away bacteria.

It will not have any negative effect on the plants. But it can affect fish that eventually cause their death.

The media in the lower layers is too dense with fish muck. Aerobic bacteria are unable to colonize this segment and a meaner strain of bacteria able to live in stagnant no oxygen conditions begins to thrive. Like an engine in reverse, this strain of bacteria begins to break the muck converting the Nitrate back to Nitrite.

Signs and effect of Nitrite poisoning in fish

Symptoms of nitrite poisoning
It’s important to watch the color of your fish. Blushing is a pink to reddish cast on fins and tail of a fish indicating stress, often caused by nitrite poisoning.

The symptoms of nitrite poisoning are pretty easy to detect. Stressed fish struggle to keep alive by frantic gill movement, loss of balance and frantic effort to suck more oxygen into their system by gulping on the surface of the water. Healthy fish are sprightly and dart around the tank at speed when startled. Fish that exhibit Nitrite toxicity will be sluggish and exhibit some of these telltale signs.

  • Fish begin to gasp for air at the water surface.
  • Tan or brown gills
  • Fish stay limp and listless near water outlets.
  • Rapid gill movement
  • Fish hang near water outlets

Nitrite toxicity is also known as Brown Blood Disease. The blood in the fish turns brown because it cannot absorb the oxygen into the bloodstream. Fish become weak and begin to suffer. Nitrite poisoning limits blood’s ability to carry oxygen, which can suffocate fish even when there is enough oxygen in the water.

Exposure to Nitrite for prolonged periods of time will make the immune system of fish weak. That eventually deteriorate in condition as secondary diseases begin to make an appearance. Fin rot, ich, and bacterial infections suddenly occur resulting in a spiral string of sudden fish deaths.

Acceptable nitrite level in the aquarium?

Graph of ammonia levels
Graph of ammonia levels.

The nitrite level should always be zero, or as close to zero as you can get it. Under certain conditions, even relatively low nitrite levels of 0.25 mg/l may be enough to weaken sensitive species. Anything above 0.1 mg/l is viewed as an unacceptable and potential cause of stress, although some fish might tolerate high levels.

How to lower nitrate levels in a freshwater aquarium?

  • Change the Water: Immediately do a 25-50% water change with dechlorinated water and continue to do this daily until your tank is cycling. (i.e. ammonia and nitrite are at zero)
  • Add salt: The addition of a half ounce of salt per gallon of water will prevent methemoglobin from building up. It will reduce the influence of the Nitrite to strip oxygen from the blood cells. Research suggests that 1lb per 150 gallons of water is an effective dosage. Salt does not evaporate out of your tank, so be careful to write down when and how much you added.
  • Reduce or stop Feeding and add no new fish to the tank until its ammonia and nitrites come to its normal levels.
  • Remove uneaten food and Clean the tank: Eliminated any food found at the bottom of the tank.
  • Increase aeration: If necessary an extra aerator with plenty of air bubbling in the water will help keep the fish alive. It will be good for beneficial bacteria as well.
  • Add Bacteria if you have changed filter, media or tank. You can do it by putting some matured filter media and/or gravel from a matured tank. This will hopefully impregnate your new tank with the beneficial bacteria. you have to be careful that what you use is clean and free from pathogens, of course.
  • Avoid using medications, as many medications kill off beneficial bacteria. Your fish may get fungus or other infections due to the stress of the ammonia and nitrite but the priority is to get that water quality as good as possible.


Your system will stay fine and healthy if you keep your system clean and do not overfeed your fish. Regular small feeds throughout the day are preferable to sudden large feeds.

Choose a hardy fish species for aquaponics capable of tolerating bad water quality. Check this guide to choosing the fish for aquaponics.

Do not overpopulate the fish in your tank. Follow the fishto water ratio guidelines to maintain the system balance.

Make sure you have enough bio-filtration (grow beds) to strip impurities out of water when recirculating the water. The water should never look foamy and cloudy.

Your water pump should be capable of turning over the water in the fish tank at least four or five times per hour in volume capacity.

Test the water for Nitrite on regular basis. A $40 dollar API Master test kit will provide all the equipment necessary to conduct the test. Five drops of Nitrite test solution in a vial of fish water will instantly tell you the condition of your water by the color. A healthy system will turn blue. A bright scarlet color spells trouble ahead.

Let the bacteria colonize the surface before using grow lights emitting ultraviolet rays. Read more about parameter affecting the aquaponics cycle here.

If you’ve got a new tank or filter, don’t add too many fish or much food. The more pollution entering the tank the more dangerous, the conditions will become for any fish present. Read more about cycle the fish tank here.

Moreover, grow media that is washed in tap water or replaced with new, contains few bacteria and aren’t capable of removing much pollution. Only wash filter media in old tank water and always replace media bit by bit, not all at the same time. To speed-up growth of bacteria, transfer some media from an existing filter, or add bacterial starter culture and food source.

You can also try some unconventional way like using mussels in aquaponics to purify water for nitrogen because they feed on algae and remove other nutrients from the water.


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