Nitrate Poisoning in Freshwater Aquarium Fish

A Freshwater Aquarium
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The danger of excessive nitrate in aquariums is often misunderstood by aquarium hobbyists. Although far less toxic than ammonia or nitrite, high nitrate levels—called nitrate poisoning or nitrate shock, can also kill aquarium fish. Chemically, nitrate is similar to nitrite, in that both consist of molecules of oxygen and nitrogen, but nitrate is a less reactive compound.

Nitrite contains only two atoms of nitrogen for each one atom of oxygen, making it a more reactive molecule, while the less reactive nitrate molecule contains three nitrogen atoms for each atom of oxygen. Nitrate is one of the natural byproducts of fish wastes and the end result of the ammonia-nitrite-nitrate cycle, and will build up in any tank that is not properly maintained.

Nitrate Poisoning vs. Nitrate Shock

Depending on the intensity of exposure, nitrate toxicity has two separate names: Nitrate poisoning refers to the chronic problem while nitrate shock refers to the acute problem of rapid onset.

What Is Nitrate Poisoning?

Nitrate poisoning occurs when fish are exposed to gradually rising nitrate levels over any period of time when regular tank maintenance is not performed. Overfeeding and overstocking of fish are also significant contributors to slowly rising nitrate levels.

When steps are not taken to reduce nitrate, the cumulative effect can be fish death. Some fish will be affected by levels as low as 20 mg/L while others will show no apparent symptoms until levels have reached several hundred mg/L. Juvenile fish are affected at the lower levels, as are saltwater fish. For most aquariums, it is recommended to keep the nitrate level below 20-40 mg/L by performing regular water changes.

What Is Nitrate Shock?

Nitrate shock occurs when fish are suddenly exposed to a vastly different level of nitrate, up to several hundred mg/L. This occurs when fish are transferred from one aquarium with normal nitrate levels to another with excessive levels. Although nitrate shock usually refers to a sudden increase in the level of nitrate, fish can be equally shocked if nitrate levels suddenly drop dramatically.

Symptoms and Mortality

These new behaviors in your fish can be signs that they may be dealing with a new nitrate issue, originating in your tank over the last 24–48 hours:


  • Loss of appetite
  • Rapid gill movement, high respiration rates
  • Listlessness, acting dazed
  • Loss of equilibrium, disorientation
  • Laying on the tank bottom
  • Curling head to tail (advanced stages)

When nitrate levels have risen slowly over time, it is not unusual for only one or two fish to exhibit symptoms initially. Until water tests are performed, there may be no apparent reason why some fish mysteriously become ill. If nitrate levels are not reduced, more fish will begin to show symptoms. Eventually, fish death will begin, occurring over a period of a few days to a few weeks.

When fish are suddenly exposed to very high nitrate levels, they will usually die within 24 hours of exposure. Often owners are not aware of the problem until the fish are dead or near death. In these situations, emergency measures can be taken for affected fish, but recognize and remove the source of the problem to avert further tragedy before new fish are added to the tank.

Most cases of sudden nitrate shock and subsequent fish loss occur when bringing new fish home to an aquarium that has drastically high nitrate levels. New fish are often chemically shocked by poor water quality conditions that current residents may have become slowly accustomed to over time. Nitrate shock can also occur when massive water changes have been performed on a mature tank that has high levels of nitrate, as any sudden drop in nitrate will also shock the fish.

Emergency Treatment

Most of the time fish will not die suddenly with gradual exposure to high nitrate levels. In cases of sudden exposure to high nitrate, it is possible to reduce the effect of the nitrate toxicity, thus giving the fish a fighting chance at survival. Perform a water change and test the ammonia, nitrite and nitrate levels in the water. The ammonia and nitrite should be zero and nitrate less than 10-20 mg/L. Methylene blue medication can be added to the aquarium water if the ammonia or nitrite levels were also high. Do not feed the fish for 24 hours, and then feed only sparingly until the water quality stabilizes.

How to Bring Down Nitrate Levels

To bring down nitrate levels, slow, controlled water changes will be necessary. The first step to a successful water change is to test the water to get a baseline nitrate level. This will give you an idea of how many changes may be necessary. Quick-test strips will provide a rough idea of your progress, but test kits using chemical reagents will provide more accurate readings.

The key to safely changing out bad water during a poisoning event is to avoid another sudden change which will provide too much chemical gradient stress on already stressed fish. Ideally, nitrate levels in a freshwater aquarium should be kept below 20 mg/L. However, any changes should occur slowly, only removing less than 50 mg/L of the nitrate per day.

If ammonia and nitrite are low and only nitrate is elevated, perform multiple small water changes. The maximum speed of concentration change that freshwater fish can tolerate is a five percent water change per hour or two, using water that is low in nitrate. Repeat changing only this amount until you have replaced approximately half of the total gallonage of water in your aquarium. This process will reduce the nitrate levels significantly, but slowly enough to avoid the effects of sudden changes on the fish.

Continue to Re-Test Levels

After the water change, re-test to find out how much the nitrate level has dropped. If the nitrate level remains above 100 mg/L, repeat the process the following day. Keep changing the water until you have reduced the nitrate level to 20 mg/L or less. Nitrate-removing filter media can also be used if nitrate levels remain high, usually from leaching out of filter media, substrate, or plants.

Preventing Nitrate Poisoning

Once nitrate levels have been reduced to normal, it is important to maintain the aquarium water properly to avoid another nitrate crisis. To prevent toxic nitrate from creeping up:

  • Ask about the recommended nitrate values of any new species. Before purchasing a new species of fish, test your water and make a note of the pH and nitrate levels. At the aquarium shop, ask the clerk to test the water to verify that the pH and nitrate levels are reasonably close to what is found in your home tanks. If the numbers aren’t close, don’t purchase fish until you can adjust your home aquarium to more closely match the lower levels of the source tank the fish has experienced in the store.
  • Keep live plants in the tank. Adding live plants to your tank is a great long-term preventive measure. Live plants utilize nitrate and actively remove it from the water, thus helping to keep nitrate levels down.
  • Feed less. If your fish are being overfed, their normal digestive processes will add excessive nitrate to the water. Make sure to feed just enough to keep the fish healthy. Nitrate levels should maintain naturally once feeding practices are corrected.
  • Do not overstock your aquarium. More fish = more waste = more nitrate.
  • Keep the tank clean. Good overall tank maintenance goes a long way toward keeping nitrate down. Remove all uneaten food promptly.
  • Increase aeration. Tanks with higher nitrate are usually also too low in necessary dissolved oxygen.
  • Perform regular filter maintenance. Regular filter cleaning and replacement of carbon filter media will help to keep nitrate at bay.
  • Use nitrate-removing filter media. Whenever nitrate levels are a persistent problem, nitrate-removing filter media is a helpful tool. This special filter media is available at many aquarium stores and can be used if other methods are not working to reduce nitrate levels sufficiently.
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  1. Aquarium Water Quality: Nitrogen Cycle. Florida Department of Agriculture and Consumer Services.

  2. Nitrate and Nitrite Poisoning in Animals. Merck Veterinary Manual.

  3. Davidson, John et al. Comparing The Effects Of High Vs. Low Nitrate On The Health, Performance, And Welfare Of Juvenile Rainbow Trout Oncorhynchus Mykiss Within Water Recirculating Aquaculture SystemsAquacultural Engineering, vol 59, 2014, pp. 30-40. Elsevier BV, doi:10.1016/j.aquaeng.2014.01.003