Nitrate Poisoning in Fish

What Is Nitrate Poisoning in Fish?

Nitrate poisoning in fish is caused by a slow buildup of the compound in aquarium water. Clinical signs of nitrate poisoning include lethargy, respiratory distress, and sudden death. Nitrate poisoning can easily be avoided by routine water quality testing.

What Is Nitrate?

Nitrate is caused by the completion of the nitrogen cycle, which converts the primary fish waste (ammonia) into a less toxic form (nitrate).

Ammonia poisoning can quickly cause death in fish, but nitrate is fairly well tolerated because it’s harder to transport through the gills and into the fish.

Freshwater fish are typically more tolerant of high nitrate levels than saltwater fish, but sensitivity will vary significantly by species. There are no external indicators of high nitrate levels, meaning you can only distinguish this by testing your water chemistry with an aquarium test kit.

Nitrate is typically used by algae as a food source in both saltwater and freshwater aquarium systems. In addition to causing nitrate toxicity in fish, algal blooms or overgrowth may also be noticed by fish parents. It can be free-floating or attached to your tank and decor.

High algae levels can be dangerous to fish overnight when algae switches from photosynthesis to cellular respiration, using up the available oxygen and putting out carbon dioxide, which can cause a pH crash.

Live aquarium plants can remove some of the nitrates in your tank, but they will reintroduce ammonia back into your system as they die off, causing the cycle to start again.

Symptoms of Nitrate Poisoning in Fish

Nitrate poisoning develops slowly, with clinical signs easily being missed or dismissed.

Signs of nitrate poisoning in fish include:

• Lethargy

• Decreased appetite

• Skin irritation/redness

• Increased respiration

• Sudden death

Causes of Nitrate Poisoning in Fish

Nitrate levels will slowly build up in your tank over time if you do not perform regular maintenance or trim up your dying plants efficiently.

How Veterinarians Diagnose Nitrate Poisoning in Fish

Nitrate poisoning can appear to be similar to many other clinical diseases, such as gill parasites or issues with water chemistry including high ammonia, nitrite levels, or inappropriate pH or temperature.

It’s critical that your veterinarian test your water quality at the time of your appointment with a liquid-based test kit. Strip testing is not a reliable method of diagnosing water quality issues.

Bring a water sample to your veterinary appointment in a sealed container, separate from the container in which you transport your fish.

Normal levels for nitrate in an aquarium vary based on the species. Most pet species can tolerate a level up to 20 mg/L, but some species may be less tolerant and will start getting sick at lower levels.

Your veterinarian will test all parameters of your nitrogen cycle to determine if there is another issue with your tank or filtration.

How To Treat Nitrate Poisoning in Fish

The only way to treat nitrate poisoning is to perform water changes to remove the nitrates from your tank.

Once you test your water chemistry and confirm the high level of nitrate, do a water change to bring the level down. This involves removing some of the tank water—no more than 50% at a time—and replacing it with new water.

Starting with a 50% water changes will significantly decrease your levels and your fish will start to feel better immediately, but it may take a few days to a week for them to start acting like their normal selves.

If you are using tap water, add a water conditioner or dechlorinator to ensure your water is safe for fish. Test your water’s pH and temperature to ensure they are close to your new water.

If your pH is very low, secondary to a low KH or carbonate alkalinity, you may be dealing with old tank syndrome and will only be able to do small water changes for a time.

For more effective water changes, use a gravel siphon to remove debris from your aquarium’s substrate.

How To Use a Gravel Siphon

1. Once the water is flowing out of the tube by suction or gravity, insert the siphon into your substrate until it touches the bottom of your tank.

2. Allow the substrate to flow at least one quarter to halfway up the rigid tube and then lift the vacuum to break the siphon.

3. Your substrate will fall back down, and any lighter particulates will continue to flow up and out along the tube.

4. Continue to work your way along your substrate until all the rocks, gravel, or sand have been cleaned.

This is the most effective method for cleaning any aquarium because it removes all the small particles that have sunk into your tank’s substrate.

Recovery and Management of Nitrate Poisoning in Fish

The best and easiest method of treating nitrate poisoning is preventing it altogether. All fish parents should test their aquarium water quality at least monthly, with weekly testing if any new fish, plants, or invertebrates have been added; diets have been changed; or filtration components have been altered.

Do not rely on test strips, and stick to a liquid-based test kit that is used regularly per the manufacturer’s instructions, stored indoors, and replaced yearly.

Always write down your values so you can note small changes over time. Do not rely on your brain to remember previous readings!

If nitrate poisoning is left untreated, your fish will either die from the high nitrates building up in their blood or from a secondary bacterial infection due to the stress of trying to cope with high nitrate levels.

Tips for Preventing Nitrate Poisoning in Fish

1. When your nitrate levels are back within safe ranges, you may need to alter your water change volume and frequency to keep them within range.

2. Review your fish’s diet to ensure they are not eating too much protein, which can lead to higher ammonia output and consequentially, nitrate output.

3. Consider adding a few plants to help bring your levels down.

4. You may need to consider a larger tank if your levels are unsustainable.

5. Some water sources can have nitrates coming from tap water, so you may need to find another water source, such as bottled spring water or by using a reverse osmosis filter system, if this is the case.