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How to Lower Nitrates in a Saltwater Aquarium: 15 Easy Methods

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By Lindsey Stanton

Colorful sea anemones in saltwater tank aquarium

All healthy aquariums have nitrates in the water. It is the result of the nitrogen cycle (the process whereby nitrifying bacteria are formed). Although a small number of nitrates present in the water will not pose harm to your inhabitants, if the maximum number of parts per million (ppm) is exceeded, you will start to have trouble inside your saltwater aquarium.

All fish can tolerate a maximum amount of 20 ppm to 40 ppm nitrates before overdose symptoms will strike. It is said that fish and invertebrates can adapt to the level of nitrates in an aquarium. This is simply false and will cause health issues once a water change is completed. The chemistry of the water is important to maintain a healthy saltwater aquarium. Fortunately, this article will inform you of our top eight methods to lower the number of nitrates in your saltwater aquarium.

Understanding Nitrates

You may be wondering what exactly nitrates are? Nitrates are a product of the nitrogen cycle produced from rotting foods, fish and invertebrate waste, and decomposing plant matter. It is derived from the conversion of ammonia into nitrites until it is finally converted to nitrates.

Nitrates are less toxic to aquatic inhabitants when compared to deadly ammonia and nitrites. Both ammonia and nitrites should be 0 ppm to avoid illness and eventually death in both fish and invertebrates. Nitrates are more tolerated and can stay under an ideal amount of 30 ppm.

Some fish can only tolerate up to 20 ppm, and the hardier fish have been reported to tolerate up to 40 ppm. Death can occur if the inhabitants are left untreated for more than a few hours in severe cases and 48 hours in mild cases.

crystals of potassium nitrate
Photo Credit: Ihor Matsiievskyi, Shutterstock

Using a Test Kit

The only way to find out the number of nitrates in the aquarium is by a liquid test kit. Since nitrites are invisible compounds that cannot be seen inside of the water, testing the water for all three parameters is essential. We recommend testing the water at least once a week. You can even use a journal to document the number of nitrates in your saltwater aquarium after every test.

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Nitrate Poisoning in Fish

Watching out for symptoms of high nitrates is crucial for all aquarium keepers. The onset of symptoms can be very sudden and may catch you off guard. A healthy fish can quickly get sick if exposed to undesirable nitrate levels.

  • The fish will float on its side in a “C” shape
  • Clamped fish
  • Loss of appetite
  • Rapid gill movements
  • Lethargic
  • Unresponsive
  • Bottom sitting
  • Tail curling
  • Browning of the gills
  • Redness or wounds on the body

If your fish has four or more of the above symptoms, your fish may have nitrate poisoning. The symptoms will usually appear altogether, but it is not unheard of for a fish to experience only some minor symptoms before visible signs are presented.

Nitrate Poisoning in Invertebrates (Snails, Shrimp, Crayfish)

Nitrate poisoning in invertebrates is slightly different than in fish. Invertebrates seem to be particularly sensitive to nitrites over 25 ppm.

  • Floating
  • Loss in appetite
  • Lethargy
  • Unusual behavior
  • Attempting to escape the aquarium
  • Multiple deaths within 24 hours
  • Laying on the side
  • Bodily discoloration

Important: Corals can only tolerate a maximum of 5 ppm nitrate before they start to lose their color. If the number exceeds, they will start to die off.

nitrate on hand
Photo Credit: Criniger kolio, Shutterstock

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The 15 Methods to Lower Nitrates in a Saltwater Aquarium

1. Avoid overfeeding your inhabitants

Overfeeding is the leading cause of high nitrates in aquariums. Feed only as much food is required and consumed within several short minutes.

2. Do regular water changes

To lower the number of nitrates immediately, do a 20% water change every hour. Do not do large water changes in a short space of time; this will cause water chemistry shock, and your inhabitants will not be able to handle the rapid change in nitrate levels.

3. Gravel vacuum (Siphon)

A lot of old food, poop, and general debris can get trapped in the substrate. Purchasing a gravel vacuum will allow you to such up all the litter. Move the substrate around with the tip of the siphon to free the dirt.

4. Use a filter

Filters are effective in hosting beneficial bacteria. These bacteria will keep your nitrate levels under control and will keep them from fluctuating. All aquarium inhabitants require a filter to keep them healthy.

Aquarium with filter
Photo Credit: M-Production, Shutterstock

5. Remove deceased inhabitants

If you notice an inhabitant has passed away, you should remove them from the aquarium immediately. If they are left in the tank for a prolonged period, they will foul up the water and cause a spike in the water levels.

6. Grow a variety of saltwater plants

Plants not only make an aquarium look attractive, but they also suck up nitrates from the water effectively. Live plants will then use the nitrates to grow, thus lowering and maintaining the nitrate levels in your aquarium.

7. Allow algae to grow

Algae is great at removing nitrates from the water. However, this can cause algae to grow more rapidly, but it is still beneficial to allow a small part of the aquarium to host harmless varieties of algae.

8. Use a protein skimmer

Protein skimmers keep nitrates on the low side by slowing down the spike in water levels. Keeping a protein skimmer in your aquarium is a great way to lower and control fluctuations with nitrates.

Ghost Shrimp
Image Credit: Nicholas Toh, Shutterstock

9. Chemical additives

Most aquariums will stock a variety of aquarium medications used to temporarily treat high levels of nitrates. This is only a good method for minor cases where the compound can be bonded for a few hours while you decide on a course of action to maintain the levels further. Ask your local fish store about the best saltwater nitrate medication for your aquarium.

10. Purchase a clean-up crew

Since overfeeding and fouling debris will spike the level of nitrates, you should investigate getting fish or invertebrates that will feast off the matter. This can include nerite snails which are great for saltwater aquariums.

11. Remove uneaten food with a net

If you find you have overfed your fish or find large chunks of uneaten food, you can use an aquarium net to scoop out the pieces.

12. Understock the aquarium

Fish waste builds up nitrate levels rapidly in an aquarium. If you overstock the aquarium, nitrates can spike uncontrollably. Stay on the safe side and only keep an ideal number of inhabitants inside of your aquarium.

13. Keep the largest tank possible

Natural waters contain very minimal amounts of nitrates. This is because large bodies of water effectively dilute the water’s contaminants. This is difficult in a small body of water, and the nitrates will build up. By keeping the largest tank possible for your environment, you allow more room for error when it comes to controlling nitrates.

Small fish tank aquarium with colourful snails and fish at home on wooden table. Fishbowl with freshwater animals in the room
Image Credit: Ja Crispy, Shutterstock

14. The refugium method

This method focuses on creating a chamber to host biofilter media to regulate the water levels. It is mainly called a sump filtration system. These chambers contain excellent materials to keep your aquarium’s water clean.

15. Cycle the aquarium

One of the first mistakes novice aquarists make is not letting their aquarium establish itself for a few weeks. Do not add inhabitants to your aquarium right away. Instead, you should wait till your tank has cycled over 2 to 5 weeks. You will be able to tell if your tank is uncycled by ammonia and nitrite levels over 0.5 ppm.

wave tropical divider The Causes of High Nitrates

Saltwater coral reef aquarium
Image By: Vojce, Shutterstock

wave tropical divider


Once you establish a steady water cleansing cycle, you should not have to worry about spiking nitrate levels. The best way to ensure your nitrites are immediately lowered is by a chemical or water change. If you have a cycled aquarium and nitrate-maintaining equipment for plants, your aquarium should keep the nitrates within the tolerance range.

Doing a weekly water test will inform you of the level of nitrates in your aquarium to give you a good indication of fluctuations or increases. Lowering nitrates is important when your test begins to show over the 20 ppm threshold. Always keep an eye on your aquarium’s water conditions, and you will be rewarded with an overall healthy aquarium!

Featured Image Credit: panparinda, Shutterstock

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