Nitrate Poisoning in Aquarium Fish

Nitrate test kit by API
Nitrate Test Kit by API. Photo from Amazon


  • Names: Nitrate Shock, Nitrate Poisoning
  • Disease Type: Environmental
  • Cause / Organism: Nitrate


The danger of nitrates to aquarium fish is often misunderstood by aquarists. Although far less toxic than ammonia or nitrite, it is still possible for aquarium fish to suffer from nitrate poisoning or shock.

Nitrate poisoning occurs when fish are exposed to gradually rising nitrate levels over a period of time.

This typically happens when regular tank maintenance is not performed. Overfeeding and overstocking are also significant contributors to rising nitrate levels. If steps are not taken to reduce nitrates, 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. Immature fish are affected at much lower levels, as are saltwater fish.

Nitrate shock occurs when fish are suddenly exposed to a vastly different level of nitrate, often by as much as several hundred parts per million. Although nitrate shock usually happens when fish are suddenly exposed to a much higher level of nitrate, fish can be shocked if nitrate levels suddenly drop dramatically. As with nitrite poisoning, immature fish and certain species, such as Discus, are more sensitive to sudden changes in nitrate.


  • Loss of appetite
  • Fish become listless
  • Fish experience loss of equilibrium
  • Fish lay at bottom of tank
  • Rapid gill movement
  • Fish curl head to tail (advanced stages)

If nitrates have risen slowly over time, it is not unusual for only one or two fish to exhibit symptoms initially. Unless water tests are performed, there is no apparent reason for the why the fish mysteriously became ill.

If nitrate levels are not reduced, more fish will 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 twenty-four hours of exposure. Often owners are not aware of the problem until the fish are either dead or close to death. In these situations, there is little that can be done for the affected fish, but recognizing it can avert tragedy for other fish that might otherwise have been added to the tank.

Most cases of sudden nitrate shock and subsequent fish loss occur when bringing a new fish home to an aquarium that has drastically higher nitrate levels. Although nitrate shock can occur when moving fish between aquariums at home, that does not occur as commonly, because usually owners treat all their aquariums similarly, resulting in similar water conditions in all tanks. Nitrate shock can also occur when massive water changes have been performed on a mature tank that has high levels of nitrate. The sudden drop in nitrate can shock the fish.


  • Use nitrate removing filter media

Even in cases of sudden exposure to high nitrates, it is possible to reduce the effect of the nitrates, thus giving the fish a fighting chance of survival. The key thing to keep in mind is to not make another sudden change. Ideally nitrate levels in a freshwater aquarium should be kept below 20 mg/l. However, any changes should occur slowly, at a rate of change that is less than 50 mg/l change per day.

Test the aquarium water and record the initial nitrate level so you have a baseline to work with. Perform a 5% water change every hour or two, using water low in nitrate. Continue until you have replaced approximately half of the volume of water in your aquarium. This process will reduce the nitrate levels significantly, but slowly enough to avoid the effects of sudden changes.

After the final water change, test the water and note how much the nitrate has dropped. If nitrate levels remain above 100 mg/l, repeat the process the next day. Nitrate removing filter media can also be used of nitrate levels remain high.

Increase the aeration in the tank, as tanks with high nitrates are usually low in oxygen. Do not feed the fish for 24 hours, and then feed sparingly until the tank stabilizes. Once nitrates have been brought down, it is important to maintain the tank well to avoid another nitrate disaster.


  • Request nitrate values before buying fish
  • Keep live plants in the tank
  • Do not overfeed, remove uneaten food promptly
  • Do not overstock your aquarium
  • Keep the tank clean
  • Perform regular filter maintenance
  • Use nitrate removing filter media

The best way to deal with this scenario is to avoid it in the first place. Before purchasing fish test your water and make a note of the pH and nitrate levels. At the fish shop request testing of the water the fish are in to verify the pH and nitrate are reasonably close. If they aren’t close, don’t purchase fish until you can adjust your home aquarium to more closely match the source tank the fish is currently in.

Adding live plants to your tank is a great long-term preventative technique. Live plants utilize nitrate, thus helping to keep nitrate levels lower in your aquarium. Make sure you don’t overfeed your fish and remove uneaten food promptly. Avoid overstocking your aquarium. More fish = more waste = more nitrates.

Good overall tank maintenance goes a long way towards keeping nitrates down. Likewise, good filter maintenance will also keep nitrates at bay. If nitrates are a persistent problem, there is nitrate removing filter media available.