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Nitrate Fish Poisoning

aquarium with fish

Taking care of aquatic life can sometimes be relatively low maintenance, but monitoring the water quality in your pets’ habitat is important. Aquarium water that gets dirty or has the wrong chemical balance can be detrimental to aquatic life health. For instance, high nitrates in aquarium environments can lead to nitrate poisoning in freshwater fish, aquatic plants and invertebrates.  

The best way to avoid nitrate poisoning and nitrate shock is by testing water levels weekly so you can act quickly if you notice a drastic elevation. Maintaining great water quality also helps avoid other long-term health issues in aquatic life. Let’s talk about what nitrate poisoning is and what you can do to help prevent and treat it.  

Table of Contents

Overview of nitrates in aquariums 

When aquatic life expel waste, it initially takes the form of ammonia—a harmful chemical that can be deadly to aquatic life in high quantities. The good news is that nitrifying bacteria in your aquarium convert ammonia to nitrite. On the downside, nitrite compounds can also lead to disease and death in low volumes.  

Once the beneficial bacteria have oxidized the ammonia into nitrites, nitrogen-fixing bacteria will convert it to nitrates. While nitrates are not as inherently toxic as nitrites, and most aquatic life can tolerate nitrates in low amounts without falling ill, they can be toxic in elevated levels.  

Thankfully, there are ways to control the level of  nitrates in an aquarium. It all begins with maintaining a healthy habitat, avoiding overfeeding and stocking the appropriate density of aquatic life. If you don’t allow waste to build up in your aquarium, chances are much lower that you will ultimately end up needing to treat nitrate poisoning in your pets.  

Why are nitrates bad? 

Nitrates are not intrinsically bad for all aquatic life. In fact, at low levels, nitrates may never cause your aquatic life to experience any problems at all. High nitrates in aquariums begin to present health issues like the following.  

Stress: Changes to water conditions create stress in aquatic life, which always presents the possibility of harming their overall health. Therefore, stress is an indirect risk factor for aquatic life who are living with high levels of nitrates. 

Poor blood oxygen health: If there are elevated nitrates in the water, your fish can’t properly carry oxygen in their blood. While contact with ammonia or nitrites puts your fish at a much higher risk of this health problem, nitrate poisoning can also lead to low blood oxygen levels.  

Growth issues: Another side effect of nitrate poisoning is causing stunted or improper growth in your fish. This side effect is particularly problematic for young fish, as it can affect their health for the rest of their lives.

Motor function issues: If there are high levels of nitrates in the water, aquatic life can even see their motor abilities affected. A diminished motor function can also make it harder for them to perform basic functions like swimming.  

Reproductive complications: Aquatic life suffering from nitrate poisoning may not be able to reproduce properly.  

While your aquarium will naturally produce nitrates as a part of the nitrogen cycle, you can avoid an overabundance of nitrates. Once you understand what causes nitrates, you can easily create a regimen for limiting the amount of this compound in your aquarium.   

What causes high nitrate levels in an aquarium? 

There are a few things that cause aquarium nitrate levels to increase. First and foremost, the accumulation of waste will lead to more ammonia, that ends with an increase in nitrates. More specifically, high levels of  nitrates can be caused by the following. 

A filter that is too small for the ecosystem: If you have a large aquarium and a filter that can’t handle the water volume, the aquarium will quickly start to see the build-up of  waste.  

Too many pets: Do you simply have too many aquatic life species in your aquarium? More aquatic life means more waste, and you may not be able to reasonably maintain the water quality unless you split your pets between two or more habitats.  

Overfeeding: You shouldn’t give your fish more than they will eat in about 1 to2 minutes. Uneaten food will fall to the bottom of the substrate and become nitrates. Fish rarely eat food that has become old or beginning to decay. 

A malfunctioning or dirty filter: Your filter is the life support system to the aquarium, and it will stop collecting waste and not perform as intended if it is not properly maintained. Conversely, cleaning your filter too vigorously may eliminate the nitrifying bacteria needed to break down ammonia, nitrites and nitrates in an aquarium.  

What should nitrate levels be in a freshwater tank? 

An ideal level of nitrates in your aquarium is 20 parts per million or less. You may not see symptoms of nitrate poisoning in your aquatic life until the level reaches 100 parts per million. However, any measurement of more than 20 should be seen as a red flag and may start negatively affecting the health of your aquatic lifes’ immune systems, energy levels, appetite and overall body condition.

How to lower nitrate levels  

If your nitrates are measuring at above 20 ppm, you should take some immediate steps. 

Perform a controlled slow water change: Nitrate levels in a freshwater aquarium should be kept below 20 ppm. If a controlled slow water change is required to address excessive nitrate levels, changes should occur very slowly over the course of a day and involve only removing less than half of the nitrates per day. 

Clean your aquarium filter: As mentioned, a clean filter is one of the best ways to combat a dirty aquarium and lower high nitrate levels. Aquarium filters are designed for a specific range of water volume. Make sure your filter is appropriate for your aquarium size. 

Check the filter for obstructions: Is your filter working correctly? If your aquarium is consistently dirty, you may need to add new media to the filter or check for obstructions.  

Clean algae and debris: Is that stone bridge in your habitat covered in a layer of green fuzz? Then it’s time to clean your aquarium’s walls, substrate and décor. You can also grab a net and clean up any floating debris or waste. 

Add live plants: Live plants help keep ammonia levels in check, as they use nitrates and thus remove them from the water. Your local Pet Care Center aquatic specialist can advise you about which plant is right for your aquarium  helping to reduce high nitrates. The specific live plants they recommend can be a good long-term preventive measure. 

Feed your aquatic life less: If your nitrate levels are high, start evaluating how much food you provide your pets at mealtime. Cutting down on extra food they won’t eat can help eliminate a significant portion of your aquarium waste.  Fish should only be fed however much food they can consume within 1 to 2 minutes. Excess food should be removed from the aquarium immediately.

Use zeolite as a filter media: Zeolite is popular  because it is very porous filter media designed to absorb ammonia  from your habitat’s water. Zeolite will need to be  replaced when it is exhausted. 

Continue to test the water: Quick-test test strips are available in Petco Pet Care Centers and will give an idea of your progress. These strips are especially helpful if a slow water change is required. More accurate test kits using chemical reagents are also an option. 

Symptoms of nitrate poisoning

Nitrate poisoning may initially present itself as lethargy. Have your fish suddenly stopped swimming around? Are they lingering near the bottom of the aquarium on their side? Fish suffering from nitrate poisoning may appear to be floating or lying on the substrate on their sides—otherwise known as being laterally recumbent. Nitrate poisoning or shock may also cause a loss of appetite, rapid gill movement and body curving from head to tail. Nitrates can also lead to red blotches or sores on their fins. Keep an eye out for changing color on the fin—this is often a sign of illness.  

Treatment for nitrate poisoning in fish

Your veterinarian or Petco Pet Care Center aquatic specialist will most likely recommend environmental changes to help aquatic life with nitrate poisoning. You may need to clean your aquarium filter more often or find a filter that is a more appropriate size. Partial weekly water changes—often up to 50% at a time—may also be a part of the treatment plan. Performing regular water changes of 10–25% every 2–4 weeks will help to maintain proper water quality and prevent elevated nitrate levels.

You may also need to add additional nitrifying bacteria to the water or provide better biofiltration. Including more live plants in your aquarium can also be a part of the treatment plan for high nitrates, as they often act as natural filters and can foster beneficial bacteria.  

How long can aquatic life live with high nitrates? 

In rare cases, some aquatic life species can die within a day or two if they are suddenly introduced to a high-nitrate environment. More commonly, your aquatic life will become sick over several weeks as the nitrate levels rise. Even at harmful levels, you may be able to save your aquatic life if you fix the root cause of the nitrate increase and seek help from your vet or local aquatic specialist. Consider upgrading to one of the newest freestanding fish tanks if you have consistent problems with rising nitrate levels.  

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Reviewed by Don Spaeth, Petco’s National Aquatic Care, Education and Programs Manager

Don is Petco’s National Aquatic Care, Education and Programs Manager. He is an avid aquarist who has worked with and cared for freshwater and marine aquatic life for over 40 years. Throughout his 27+ years with Petco, Don has actively been involved with our aquatic vendor partners and worked to promote aquatic education both in store and company-wide.