What Causes High Nitrate Levels in Fish Tanks?

by Aquarium Scoop | Last Updated: February 3, 2023

One of the most important things to remember when owning a fish tank is that high nitrate levels can harm your fish. Nitrates are produced by decomposing organic matter and can build up quickly in your tank.  

In this guide, you’ll learn:

  • What are nitrates?
  • Why is it important to keep nitrate levels low?
  • What can cause high levels of nitrate in your aquarium?
  • What can you do to keep nitrate levels low in your fish tank?

What are nitrates?

Nitrates are chemical compounds made up of hydrogen and nitrogen. They’re a byproduct of ammonia, a gas produced when organic matter decomposes. 

Nitrates and ammonia are made in fish tanks when fish waste and uneaten food break down. 

Therefore, nitrate levels should be checked and controlled often to ensure that fish and other aquatic life live in a healthy and safe environment.

Expert Tip 🧠

You can measure nitrate levels in your tank by purchasing nitrate water test kits online or at your trusted pet shop.  

Why is it important to keep nitrate levels low?

Nitrates in fish tanks aren’t harmful in and of themselves, but when there are too many of them, it can cause several problems. For aquarium owners, keeping the right balance of nitrates in the water is essential for the health of the fish and other aquatic life. 

High levels of nitrate in your aquarium can:

High nitrate levels in fish tanks can be a severe problem for fish and aquarium owners alike, so understanding what causes high nitrate levels in your fish tank is essential. 

Expert Tip 🧠

Nitrates are harmful to fish above 50 ppm and can be lethal above 100 ppm. An average level would be anywhere from around 0 ppm to 25 ppm. 

What can cause high levels of nitrate in your aquarium?

Nitrate levels should be checked and controlled often to ensure that your fish and other aquatic life live in a healthy and safe environment.

Here are three of the most common causes of high levels of nitrates in your tank:

  1. Overfeeding your fish
  2. Broken aquarium filter
  3. Not cleaning the tank regularly

Overfeeding your fish

If you have a fish tank, you must be aware of the dangers of overfeeding your fish. When fish are overfed, they produce more waste, which raises the level of ammonia and nitrates in your tank’s water and may cause your fish to become sick or even die. 

Broken aquarium filter

When an aquarium filter isn’t working right, the amount of nitrate in the aquarium can rise quickly. This is because the aquarium’s filter can’t get rid of the nitrate, so it builds up over time. 

In addition, if the filter is not working correctly, it can also cause an increase in bacteria, which can further contribute to nitrate buildup. As the nitrate levels rise, the fish and other animals in the aquarium may be in danger. 

Not cleaning the tank regularly

The nitrate level can become dangerously high if your aquarium is not cleaned regularly. This is because of the buildup of organic matter, like fish waste and food that hasn’t been eaten. This organic matter breaks down into nitrates, which the water then takes in. 

Clean your aquarium often to keep the nitrate level at a healthy level.

📚 Read More >> How To Get Crystal Clear Aquarium Water

Expert Tip 🧠

If you think you are reading your nitrate test kit’s result wrong, it always helps to do another test. Make sure to shake the nitrate test bottles thoroughly and follow the instructions exactly. 

What can you do to reduce nitrate levels in your fish tank?

There are several things that you can do to lessen and keep nitrate levels low in your fish tank:

Cycle your new fish tank

If you just bought some fish or want to start a freshwater aquarium, you need to know how to cycle a new tank. Cycling helps make the water safe for your fish by adding good bacteria that break down waste before it can build up and become harmful. 

Cycling a new tank can take up to eight weeks, but it’s worth it to ensure the health of your fish. 

During this time, you’ll need to test the water regularly and do regular partial water changes as required. Then, once the cycle is complete and ammonia and nitrite levels have stabilized, you can add fish to your aquarium.

By cycling your tank before adding any fish, you can be sure that there won’t be any sudden spikes in ammonia or nitrite levels that could harm them.

Avoid overfeeding your fish

Overfeeding can cause excess waste, which can raise nitrate levels. Try feeding smaller amounts more often instead of one large meal per day.

Prevent overfeeding by measuring how much food your fish need using a guide or chart. A good rule of thumb is to feed your fish only as much as they can eat in 2-3 minutes.

Once you’ve determined how much food your fish need, be sure not to follow the “one size fits all” approach. Some days, they may want less food than usual, while others may be hungrier than average. Pay attention to your aquatic pet’s eating habits and adjust accordingly.

Make sure your filter is working correctly

Regularly check your aquarium filter and make sure it’s functioning correctly. You also need to ensure you have the correct filter size and type for your tank, or else it will need extra help to do its job!

Regularly clean your tank and change its water 

Clean out your aquarium tank at least once a month. Depending on the size and type of your aquarium, you may need to clean it more often. A partial water change (25-30%) every 2-4 weeks is also recommended. 

Do regular water changes and vacuum the gravel to remove uneaten food and waste. This will help keep the water clean and reduce the amount of ammonia in the tank, which can lead to high nitrate levels.

Keep live plants in your aquarium

One way to help keep nitrates under control in your tank is by keeping live plants in it. While chemical treatments are available to lower nitrate levels, live plants are a meaningful natural way to keep them under control. Plants help to keep nitrate levels low by absorbing toxins from the water.

Aside from that, live plants also oxygenate the water, which is essential for fish health and adds beauty and interest to your aquarium. So not only are they suitable for your fish – they’re also good for you!

Expert Tip 🧠

Make sure you research which types of plants will do well in your particular setup – some need more light or different substrate than others – and don’t forget that, like all living things, they’ll need regular care (trimming, etc.)


How do you fix nitrate levels in a fish tank that’s already high?

There are a few ways to fix high nitrates in a fish tank:

What causes nitrate levels to be high?

Many things can cause high nitrate levels in a fish tank. The following are the most common:

What does it mean when nitrate is high in a fish tank?

When nitrate is high in a fish tank, the nitrogen level in the water is too high. This is bad for fish because it can make it hard for them to breathe and cause other health problems.

High nitrate levels can also lead to algae growth, making the tank look dirty and cluttered. To avoid these problems, aquarium owners should change the water often and closely monitor the nitrate levels.


Nitrate levels should be checked and controlled often to ensure that fish and other aquatic life live in a healthy and safe environment.

High nitrate levels in your aquarium can make the water quality terrible, which can hurt or kill fish, as well as grow harmful bacteria and algae, which make it hard for plants and other aquatic life to do well.

Three of the most common reasons your tank has a high level of nitrates are giving too much food to your fish, having a broken aquarium filter, and not cleaning your tank often enough.

You can lower and keep nitrate levels low in your fish tank by cycling your new tank, avoiding overfeeding your fish, ensuring your filter is working correctly, regularly cleaning your tank, changing the water regularly, and keeping live plants in your aquarium.

Aquarium Scoop is the perfect place to start if you are new to fish ownership or have never had an aquarium. We offer a wide variety of services that can help educate you on how to take care of your fish and other aquatic life. From beginner’s tips on setting up your first tank to more advanced advice on filtration and water quality, we can help you ensure that your fish stay healthy and happy for years to come.