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Nitrates in Saltwater Aquariums

Reviewed by Don Spaeth

Clownfish hiding with Anemone

As a saltwater aquarist, you already know your aquarium is far from a set-it-and-forget-it operation. Rather, you must continually monitor your aquarium and provide necessary health and wellness solutions to keep your aquatic life happy and healthy. One of the most important components to observe to help maintain ideal water quality is the level of nitrates in your aquarium.

Though nitrates in fish tanks aren’t as lethal as other compounds—like ammonia or nitrites—high nitrates in an aquarium can stress and eventually harm aquatic life. They can also lead to algae blooms and further trouble in your aquatic habitat.

Understanding the nitrogen cycle

How are nitrates in aquariums created? These compounds are a natural part of the nitrogen cycle in your aquatic ecosystem. This process involves the transformation of three compounds—ammonia, nitrites and nitrates.  

Ammonia - The nitrogen cycle begins with the introduction of ammonia into your aquarium. Aquatic life produce waste, which is one of the largest sources of ammonia. Decaying food and vegetative material also create ammonia as they break down in your aquarium. Ammonia is highly toxic to fish and other aquatic animals.

Nitrites - Fortunately, a special type of bacteria called Nitrosomonas—also known as nitrogen-fixing bacteria—consume and oxidize the ammonia in your aquarium, converting it into a less toxic compound called nitrite.

Nitrates - Finally, Nitrobacter bacteria oxidize nitrites into nitrates. These nitrates are the end product of the nitrogen cycle and are far less harmful to fish and other aquarium occupants than ammonia or nitrites. In fact, nitrates can serve as food for various aquatic plants and algae.

Saltwater fish can tolerate some nitrates in their environment, but that doesn’t mean they are completely safe. High nitrates in an aquarium can stress your fish and even kill them if levels get too high. The ideal level of nitrates in a saltwater aquarium will depend on the species included in your habitat. However, safe nitrate levels in a saltwater aquarium are typically considered to be no higher than 20 ppm.

nitrogen cycle

What happens when nitrates get too high

Elevated nitrate levels in your aquarium can lead to the creation algae—you may even develop an algae bloom in your habitat. High nitrate levels can also lead to bacterial imbalances and hinder the growth of coral.

Increased nitrate levels can begin to stress your aquatic life. Though you might not notice any immediate issues, this stress can negatively impact your aquatic life’s immune systems and make them more susceptible to disease. They may also have difficulty reproducing. High nitrates in a fish tank will also have a significant impact on young fish and could affect their growth.

Big swings in the level of nitrates in your aquarium can be just as harmful as high levels. Aquatic life can develop a tolerance for a certain level of nitrates in the water, but a big shift can stress your pets or even put them in shock.

Eventually, nitrate levels can rise to the point that it becomes intolerable for your aquatic life. You may notice that your fish become lethargic, or you might see red marks or open sores on their skin. If these conditions are not improved, your aquatic life likely won’t survive.

How to monitor nitrate levels in a saltwater aquarium

The first step to maintaining acceptable nitrate levels in your saltwater aquarium is to know what’s happening in your pets’ ecosystem. You can find many different types of water testing kits to help you—just make sure your kit includes a test for nitrates.

Because every test works a little differently, read and follow the directions on the kit carefully. In most cases, you’ll need to compare the color on your test to a color chart to determine your aquarium’s current level of nitrates. It’s crucial to test your habitat regularly to catch problems early.

New tank syndrome

A balanced nitrogen cycle is crucial to the health of your saltwater aquarium, but you need beneficial bacteria to help propel that cycle. Remember, these bacteria are what convert ammonia to nitrites and nitrites to nitrates. When you set up a new habitat, it can take anywhere from a couple weeks to several months for these bacteria colonies to grow and flourish.

Adding too much aquatic life to your new aquarium too quickly could produce far more waste than your new ecosystem can manage. This can cause an algae bloom, as well as a dangerous buildup of ammonia.

One of the best ways to avoid this scenario is to introduce only a few aquatic life species to your aquarium at a time. You can also jump-start the nitrogen cycle by transferring substrate from a healthy habitat or adding a commercially available bacteria supplement.

How to reduce nitrate levels in saltwater aquariums

If testing shows that your nitrate levels are too high, you’ll need to get them down quickly before they seriously affect the health of your fish. One of the best ways to lower nitrates in a fish tank is to perform a partial water change.

Increasing the frequency and amount of regular water changes can help bring your nitrate levels down. However, if you continue to have high nitrate concerns after taking appropriate action steps, you will also want to test your local water supply to ensure it is not high in nitrates.

Using the right aquarium cleaning supplies to assist in cleaning your aquarium can also help lower nitrate levels. Specifically, vacuuming your substrate—where waste tends to collect—can help lower ammonia production and thus reduce nitrate levels.

How to keep nitrate levels down

The ideal way to lower nitrates in your saltwater aquarium will depend on the root cause. Here are some helpful solutions you can try:

Reduce the amount of aquatic life. The most common contributor of high nitrates in a saltwater aquarium is too much aquatic life in too small a space. Your habitat’s biological filter can only handle a limited amount of waste. If nitrate levels continue to rise quickly in your aquarium, it may be good not to replace any deceased aquatic life until your aquarium’s nitrate level is controlled.

Stop overfeeding. Another very common mistake aquarists make is overfeeding their aquatic life. Overfed pets will produce more waste, which means more ammonia, nitrites and eventually nitrates. Uneaten food will also decay and produce ammonia. A good guideline is to feed your fish only as much saltwater fish food as they can eat in one to two minutes.

Increase the size of your filter. An easy way to ensure your aquarium filters out more waste is to simply purchase a larger, more powerful filter. If you don’t have the right aquarium filter and media for your habitat size, your filter may simply be unable to keep your habitat clean enough. You’ll also want to clean out the filter regularly and replace the filter cartridge monthly, as accumulated waste in a filter can still produce ammonia.

Add live plants, macroalgae and live rock. Live rock house anaerobic bacteria and can help the conversion of nitrates to nitrogen gases, while live plants and macroalgae are eager to gobble up nitrates. They can also add beautiful visual elements to your aquarium. Just make sure to prune your plants and macroalgae—dead pieces left in a tank will decay and produce ammonia.

Specialized filters and media. One final option to address high nitrates in your aquarium is to add a protein skimmer or invest in special filters called denitrators. Protein skimmers help remove waste before it begins the nitrogen cycle, and denitrators are specifically designed to lower nitrate levels. These filters can be somewhat pricey—a lower-cost option would be to purchase filter media designed to capture nitrates that can be added to your existing filter. 

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Reviewed by: Don Spaeth

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.