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What Nitrate Levels Are Acceptable in a Saltwater Aquarium?

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In a fish tank, water quality is undoubtedly the engine that drives life, and therefore knowing what to do when pollutants build up is a top concern for hobbyists.

So how should you react when you need to lower the nitrate levels in a saltwater aquarium without causing shock to the fish and invertebrates inside?

Different tanks may have varying nitrate contents but still thrive as long as these contents range within the tolerable limits. What water test reading is within that safe range when you have a new or an established marine aquarium?

There are multiple ways to manage and control the high nitrate levels in a saltwater tank but practice shows that the best results are achieved by implementing more than one control method at a time.

So now let’s get into it to break down these.

To understand the methods, however, an explanation as to how the nitrogen cycle works is necessary.

The first phase of a nitrogen cycle involves organic waste in the aquarium being converted into ammonia (NH3), which in turn further degrades into a harmful substance called nitrites (NO2-). This is achieved by Nitrosomonas — a colony of bacteria that feeds on ammonia by oxidizing it.

The Nitrobacter, another colony of nitrifying bacteria, consumes these nitrites while releasing the less harmful nitrates as a by-product. Nitrate is the final product in the nitrogen cycle.

Since nitrates do not convert into another form of nitrogen on their own, the fish-keeper has to devise ways to reduce their levels.

Without human intervention, nitrates will continue to accumulate in the fish tank until the whole closed system is too polluted to harbor aquatic life.

What is the Acceptable Level of Nitrate for a Saltwater Reef and Fish-Only Aquarium?

Nitrate only becomes harmful to fish and invertebrates when allowed to accumulate for prolonged periods of time.

The acceptable Nitrate levels in a saltwater aquarium are defined as:

  • Saltwater aquarium fish can put up with higher levels of nitrates compared to freshwater fish, so for a marine, fish-only aquarium, an acceptable level of nitrate content would be between 10 and 40 PPM (parts per million).
  • A safe level of nitrate for invertebrates and corals in a reef aquarium would be 2 PPM since the invertebrates are generally more delicate, and therefore sensitive to contaminants. Though the more resilient corals would survive this, a reading showing above 5 ppm may prove to be too high and potentially lethal to the more fragile anemones and coral.

Author’s note: Some aquarists intentionally dose their reef tanks with more Nitrate to supposedly improve coral coloration. However, what’s important for a thriving reef colony is not the amount of Nitrate but rather maintaining the correct Phosphate to Nitrate ratio, also known as the Redfield ratio. The Phosphate content in the reef tank should be significantly higher than the Nitrate or you risk bleaching your corals. However, high Phosphate can induce the growth of unwanted algae in the saltwater aquarium.

Anyway, while saltwater fish can tolerate higher nitrate content than the amounts mentioned above, it should be noted that prolonged exposure to such environments would eventually affect their health and shorten their lifespan.

Fish deaths may come slowly, but surely and you could expect other side effects such as slowed growth, and a weakened immune system.

The damage in aquarium fish related to continuous exposure to nitrate levels above the recommended range is called nitrate toxicosis.

From my experience and observations, there are many cases of aquariums being kept by the book, with nitrates on the higher end, when suddenly the fish start to show signs of severe poisoning.

It turns out that fish can tolerate these levels for a very long time before getting ill, which is not the same as living a healthy life.

What Causes the Nitrate Levels to Rise in a Saltwater Fish Tank?

As already stated, nitrates in marine water result from degrading organic matter.

A rise in NO3– is thus tied to the amount of organic waste a closed ecosystem generates.

Organic waste itself is being emitted from various sources.

Have a look at the common causes for rising nitrate levels in a saltwater aquarium:

  • Overfeeding the fish.
    While feeding the aquarium fish is one of the most cherished pastimes that come with fish-keeping, it can become a hazard if overdone.

    In fact, overfeeding is the primary cause of high nitrate content in a recently started saltwater tank.

    When the fish are provided with more than they need, the abundant food particles become organic waste and rot on the tank’s bottom.

    Also, fish that eat a lot will evidently produce a lot more waste.

  • Overstocking the fish tank.

    Stocking the aquarium with many fish and other tank-mates adds fun to viewing, but that could also have harmful repercussions if not carried out in moderation.

    More livestock means more waste in the aquarium, which in turn piles up nitrates.

    One needs to keep a manageable fish population according to the size and water volume of the aquarium to minimize this effect.

    Another unfortunate consequence of overstocking your aquarium is that when a fish dies it can go unnoticed for days.

    You should always remove dead fish from your tank as soon as possible because the rotting corpse may cause the Ammonia levels in the water to rise and kill the remaining fish. You can check this resource on how to lower Ammonia.

  • Polluted water filters.

    If the filter media is not cleaned regularly or thoroughly enough, waste accumulates in saltwater aquariums, and this soon multiplies the nitrate levels.

  • Underdeveloped denitrifying bacteria.

    In a marine aquarium system, the de-nitrifying bacteria that inhabit porous surfaces such as live rock and live sand convert nitrate into harmless Nitrogen.

    However, it takes time and certain anaerobic conditions before a new saltwater tank could have its relatively high nitrates under control.

    The de-nitrifying bacteria are only fully developed after 6 or more months, given their environmental requirements are met.

  • Not changing the aquarium water.

    If denitrifying bacteria have not yet developed the only viable way of nitrate export remains physical removal or absorption.

    Keeping water for a long time without changing it causes nitrate levels to gradually elevate until they become a threat in a marine aquarium.

How to Remove and Control the High Nitrate Contents?

It’s crucial to learn the methods for lowering the nitrate levels in a saltwater aquarium and diligently practice them if you want to keep a healthy system in the long run.

There are several techniques to try, and you can combine them for better results.

How many techniques you apply should be determined by the level of contamination and how fast you would want to get rid of it.

To lower or altogether remove high nitrates from a saltwater aquarium you can:

1. Replace aquarium water periodically.

Regular water changing helps to get nitrates down and maintain a healthy fish and invertebrate environment in a saltwater aquarium.

However, since nitrates are ions, a sudden drop could potentially lead to impaired osmoregulation in fish.

Failed Osmoregulation from a changing too much aquarium water at once will make it difficult for marine inhabitants to regulate their body fluids.

To safely reduce the nitrate content, without causing osmotic shock to marine fish, do not replace all the water at once.

Removing a maximum of 50% per weekly session is advisable since that is not going to affect the water parameters in the aquarium adversely.

An even better approach is to carry out multiple smaller water changes of 15% of the total water volume, executed an hour or two apart.

If using tap water for the exchange, it should be tested with a nitrate test kit prior to doing the change to ascertain it has little to no nitrate content.

Tap water in the United States could contain up to 44 ppm of Nitrate.

To avoid the reintroduction of NO3– many hobbyists use Reverse Osmosis (RO) water or other filtered water, which has undergone remineralization with aquarium salts.

If done correctly and regularly, a partial water change is a very effective method to lower the nitrate levels in a saltwater reef or fish-only tank.

Having your own RO system at home is very cost-efficient when keeping a marine aquarium.

You can check one of the best units for that by clicking here.

2. Collect organic waste with a protein skimmer.

A protein skimmer goes a long way in controlling nitrates by lowering the organic waste products in a saltwater tank. The skimmer collects and eliminates the organic compounds before they are able to break down into nitrates.

In fact, the main reason that protein skimmers are so popular with aquarists is their ability to indirectly get the nitrates down in more contaminant-sensitive systems such as coral reef aquariums.

Bear in mind that a protein skimmer is not going to remove the organic waste as efficiently as you would wish if it’s not the right size for the tank’s water volume.

3. Grow macroalgae in a refugium.

The macro-algae are essential in a marine aquarium since they help to remove excessive nitrate levels as they feed on sources of nitrogen for growth. Macroalgae are best grown in a separate tank connected to your display one, called a refugium. To lower the nitrates in a saltwater aquarium quickly, you should consider a type of macroalgae that grows really fast such as the Chaetomorpha.

Put a small ball of Chaetomorpha in your refugium and provide it with an inexpensive LED grow light that mainly emits wavelengths in the red spectrum.

The red spectrum will maximize the growth factor, which in turn will allow the macroalgae to suck up more nitrate in the process.

To save you a couple of hours of research you can check this list of extremely effective LED refugium lights.

Anyway, another option is to actually make a dedicated planted saltwater tank and connect its circulation to your main display one. The pretty marine plants are usually types of macroalgae and it’s not unseen for people to only have those in their display aquarium.

4. Control the fish food offerings.

While you shouldn’t starve your fish, it is also not in their best interests to stuff them with excess food.

Feeding fish sparingly can significantly reduce the high nitrate content in your saltwater tank. This is especially important if the goal is removing excess nitrates from a reef tank where the more sensitive corals can perish from the contamination.

The emphasis is on the amount you give the fish per feeding and the frequency at which they are fed.

Feeding them small amounts too often is as problematic as overfeeding at once.

If you don’t really feel confident in your ability to provide equal doses of food each time you can opt for an inexpensive automatic fish feeder.

I personally am a fan of the battery-operated one from Eheim, which can be almost as durable as their canister filters.

If you do go that road consider getting their “food station” too.

The station is essentially a chamber that delivers the meals in the same place every time, preventing food from floating around and allowing it to soak and sink properly if needed.

5. Add organic Carbon with vodka dosing.

Vodka dosing is a relatively new concept but it has already become one of the best ways to reduce the high nitrates in reef systems due to its efficiency.

There are marine heterotrophic bacteria that take in carbon, nitrate, and phosphate as their key components when they multiply.

The idea behind this method is that when nitrates and phosphates are abundant you provide the bacteria with a source of organic carbon such as vodka or vinegar which they use up by colonizing the marine aquarium. The colony then grows exponentially, thereby lowering nitrates levels in the process.

The nitrate is not exported from the aquarium system by this method, but only gets transformed to bacterial biomass, so you’ll still need to physically remove that.

The overpopulation of bacteria will build up as a slimy film on the water’s surface.

To avoid the need for water changes this method is best combined with having a good protein skimmer in place.

The protein skimmer will collect the bacterial biomass.

For better nitrate control when using this method, you should be dosing vodka continually.

That being said, here’s how to dose vodka to a reef tank in order to reduce nitrates:

  1. Dose 0.2 ml of vodka with 40% alcohol content per 25 gallons of aquarium water daily for the first 3 days.
  2. The next 3 days increase the daily dosage of vodka to 0.4 ml per 25 gallons of water.
  3. The following week you can safely dose 1 ml of vodka per 25 gallons of water each day.
  4. From then on, each week increase the daily dose by 0.7 ml until you bring down the nitrates in the aquarium water to the desired levels.
  5. When the drop in nitrates is achieved cut the vodka dose in half and maintain that as an ongoing tempo.

6. Manage an optimal bioload for the tank’s size.

Keeping fish species in large numbers in an aquarium can be really attractive and makes the tank look active and lively.

More fish, however, raise the nitrate levels even higher since they discharge more waste.

Research each particular fish you want to keep and stock your aquarium accordingly. Again, this doesn’t directly remove the high nitrates from saltwater aquariums, but it helps to reduce the produced levels.

On the other hand, a comfortable population of fish also guarantees enough room for each of them to enjoy swimming. Overcrowding the tank also makes life harder for the easily stressed, timid fish since they hardly get private space.

7. Set up a Sulfur reactor.

Sulfur Nitrate reactors are relatively unpopular and new in the whole fish-keeping hobby.

A sulfur denitrator (reactor) will promote the growth of a specific kind of anaerobic bacteria, which are very efficient in converting Nitrate to Nitrogen molecules. For this reason, they are, perhaps, one of the most powerful ways to completely remove the nitrate content from any type of saltwater aquarium system.

Though there’s a higher initial investment for this type of equipment, it is extremely cost-efficient in the long run and the maintenance it requires is almost non-existent.

During my research, I have honestly tried to find someone with a bad experience from using a sulfur denitrator, but every story I stumbled upon was packed with success.

On top of being effective, sulfur denitrators also have a small footprint and you get to be flexible with the adjustment of nitrate contents in the aquarium water to more than 0 if you’d like.

Korallin seem to offer a solid sulfur denitrator chamber, which you can check out on Amazon.

8. Use a nitrate remover.

There are now liquid or solid media supplements designed specifically to help with managing desirable nitrate levels in a saltwater reef aquarium at home.

Bear in mind that they do not fix the root of the problem but only deal with the nitrates that are already in the tank.

Some of these products are a type of media that provides porous surfaces which create conditions with no present oxygen.

In the lack of oxygen denitrifying bacteria thrive.

The liquid Nitrate removers are usually a form of organic carbon and work excellent when combined with protein skimmers in any marine reef aquarium. Using either solid or liquid nitrate-removing products is one of the best and easiest ways to get excess aquarium nitrates down quickly.

Follow the instructions on these carefully because some of the media-types such as Seachem’s De-Nitrate require a low water flow rate in order to harbor anaerobic bacteria.

If an optimal flow rate is achieved and maintained with these media, you can expect excellent results to ensue very quickly.

False Readings are Possible with Certain Test Kits

At times inefficient kits have misled fish-keepers into believing their aquariums are overloaded with nitrates when the exact opposite is true. This usually occurs when the nitrogen cycle is only halfway through and so the nitrite (NO2-) levels are higher than the nitrate (NO3-) itself.

Often the false reading is produced from a new saltwater tank where the kit may report high nitrates when in truth the tank is only high in Nitrite.

It all has to do with the way many of these kits are designed to operate.

They first reduce nitrate into nitrite and then react with the latter. It’s this reaction that gives the faulty results.

If the water has elevated Nitrite content, the reaction could be overwhelming, giving the wrong impression that the nitrate levels are high. Salifert Nitrate is one such test kit that may unintentionally mislead if you do the testing in the early phase of cycling an aquarium.

The false reading could show tens if not hundreds of times more nitrate content than the actual one in the presence of nitrite.


Remember that there is rarely a remedy that’s going to single-handedly solve the problem in your saltwater fish tank.

Combining methods is the surest way to avoid the possibility of nitrate poisoning which, as discussed, could be catastrophic.

Nevertheless, a saltwater aquarium is not that difficult to take care of once you get basic procedures like controlling nitrate levels right.

Keep learning and leave me a comment below if you have any questions.

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