How to Lower Nitrates in a Saltwater Aquarium: Proven Techniques For Success

Keeping nitrate under control has always been a concern with reef aquarists because nitrate is the end-product of nitrification via your biological filtration so it’s continually added to your aquarium water. For many, It may seem like a constant battle to keep nitrate under control. But the good news is there are many proven ways to minimize nitrate build-up. So here’s what you need to know to take control of nitrates in your reef tank!

First, let’s talk about about nitrate and how it affects your tank inhabitants. Nearly all the scientific research about the effects of nitrate in an aquarium has been conducted with freshwater fish and invertebrates. But these results can’t be applied to saltwater fish and inverts, specifically corals. What little information is available seems to indicate nitrate is not that harmful, at least when compared to ammonia and nitrite.

Studies performed with reef-building corals, Porities and Montastrea, demonstrated that nitrate levels as low as 0.3 ppm stimulated zooxanthellae growth within the coral tissue. This resulted in slower coral growth, presumably due to the algae out-competing the coral for reef-building carbonates. Another study using Acropora indicated that nitrate didn’t affect coral growth at all. A study on pink shrimp concluded that nitrate should be kept below 200 ppm.

From this we can conclude that sensitivity to nitrate is very species specific and Science so far shows that nitrate is not a deadly killer of corals but long-term, elevated nitrates may inhibit certain corals from growing at full potential.

Excess Nitrate and Phosphate levels can contribute to algae growth

There is, however, another more concrete reason to keep an eye on nitrate levels. Nitrate can stimulate algae growth. Nitrate is a great water quality indicator because as nitrate rises, so do other organic compounds that will contribute to algae growth and negatively impact your fish and corals. 

The recommended nitrate levels in a reef tank can be confusing, you will likely get different answers based on who you talk too or what you read. The facts are nitrate levels on a wild reef in the ocean is below 0.1 ppm. If we make this the benchmark, nitrate levels should always measure zero using home aquarium test kits. Our captive reef tanks are drastically different than a wild reef, however, considering that many successful reef aquarists keep beautiful, thriving corals with Nitrate levels above 10 ppm. So, the general idea for Reef Aquariums is to keep nitrates as low as possible, without stressing over a specific level. Keeping the nitrate at 2 ppm or less seems to the norm.

The most convenient way to keep track of nitrate is with home water test kits. The key to obtaining accurate results is to follow the instructions exactly as written. Mail in water test kits are another option for testing nitrates. It does take more time to get the results, but you’ll also get a full analysis of your aquarium water along with it.

Now that we know a bit about nitrates in general, let’s move onto how to control them. Water changes are great for diluting organics and re-balancing salts and trace elements, but they don’t work so well for controlling nitrates. The typical 10 to 20 percent water change, even if made every week, will never be able to significantly decrease nitrates. That’s because nitrate is constantly being produced by the biological filter from food and fish waste. The dilution factor is just too small to make a dent in the nitrate level and there are certainly more effective ways to remove nitrates with less effort.

As with most aspects of aquarium keeping, an ounce of prevention is worth a pound of cure. If the water you use to make saltwater and top-off the tank contains nitrate, you’re making things worse. You can avoid this problem with a good reverse osmosis system such as the MD Kleanwater units. RO systems will remove nitrate and a long list of undesirables you don’t want to add to your reef. It’s the same with feeding and stocking levels. The more food and fish that you add to the tank, the more nitrate will be created. Feeding and stocking is fun, but too much will certainly get you into trouble fast. This is the exact reason that when keeping a reef tank with corals, it is a good practice to minimize the fish and strictly adhere to proper feeding techniques.

Setting up a water change station is a great way to ensure there are no nitrates entering your tank from the water supply

Biological nitrate control is an important foundation for any saltwater aquarium. The process is simple, bacteria convert nitrates to nitrogen gases, through a process called denitrification via anaerobic bacteria. It happens naturally in tanks containing live rock. The tiny crevices and pores are the ideal location for denitrifying bacteria that require low-oxygen conditions.  For many reef tanks, this is all that’s needed to keep nitrate levels low alongside an appropriate fish stocking level and maintenance practices.  

In cases in which nitrates are consistently a problem, alternative methods of control can be employed.  Liquid nitrate removers, in their many formulations, take advantage of beneficial bacteria in a special way.  They provide carbon, that fuels the growth of denitrifying bacteria and when anaerobic bacteria is growing at accelerated rates, even more nitrate will be converted and removed from your aquarium water.

Liquid carbon can also force other types of nitrogen-using bacteria to proliferate.  These nitrate fixing bacteria can then be removed via protein skimmers.  Protein skimming will strip-out the nitrogen-rich bacteria from the water, ultimately reducing nitrates.This process of dosing a liquid carbon source is often called “carbon dosing”.  Red Sea Nopox, AZ-NO3, Brightwell Aquatics Reef BioFuel, and sometimes even vodka are all examples of carbon dosing.

Think of a refugium as a mini aquarium tucked under your main tank. It can simply hold more live rock for biological denitrification but will typically also house macroalgae with the appropriate lighting to support it.  Macroalgae uses nitrogen for growth and therefore when grown in a refugium, nitrate will be removed from water as the algae grows. When the algae is harvested from the refugium, the nitrate comes with it.  This same principal applies to an algae reactor such as the Pax Bellum units, macro algae grows inside the reactor consuming nitrates as it grows.

Growing macro algae in a refugium helps to remove nitrates and phosphates from your system

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Algae scrubbers operate on a similar concept but rely on a different type of algae.  Water flows through colonies of microalgae that are “corralled” inside a scrubber chamber. The microalgae remove nitrate, incorporating it into their cells. It’s important to then periodically harvest the algae to physically remove the nitrogen from the aquarium.

Nitrate Removal medias come in different forms, classically these medias rely on biological activity in order to consume nitrate but recently Blue Life has presented us with a regenerable adsorption resins that specifically targets nitrates and only nitrates.

Granular nitrate removers are made from porous, ceramic-like materials. The porous structure and rough surface provides an environment suitable for denitrifying bacterial.  Think of this media like tiny live rock. It will typically be placed in a media bag or inside a reactor and keep in mind it takes several months for the media to become colonized with bacteria and reduce nitrate levels.

Biopellets are a solid form of organic carbon, designed to slowly feed denitrifying bacteria inside of a reactor.  The pellets provide a food source and a place for the bacteria to grow on.  The pellets are tumbled heavily to constantly sluff the bacteria growth which then exits the reactor and is removed via a protein skimmer.  This process is very similar to carbon dosing but is done inside a reactor.  Proper water flow through the reactor and a very efficient protein skimmer are crucial for this method to be effective.  

Nitrate adsorption resin is a recent advancement thanks to Nitrate Fx from Blue Life USA which specifically absorbs nitrate only from your aquarium water.  The media can be expensive when used for constant nitrate control but it can also be regenerated for multiple uses which helps combat the higher price. Regeneration is simple and involves soaking the media in a brine solution.  The media is very tiny so it is important to contain it inside a media bag so you don’t accidentally release the tiny beads into your display.

A sulfur denitrator is a recirculating reactor that houses a bed of sulfur media below a bed of calcium carbonate media.  By creating an anaerobic chamber inside the Bio-Denitrator, a colony of nitrate-consuming bacteria is established on the sulfur media. The bacteria converts sulfur into sulfate and nitrates into nitrogen gas. At the same time hydrogen gas and CO2 gas are also produced: which lowers the pH of the water inside the reactor.  To help offset the drop in pH, a layer of calcium media is placed above the sulfur media which will help increase pH and also enrich the water with calcium before it exits the reactor.  These sulfur nitrate reactors can be very useful for heavily stocked aquariums in which extremely high nitrates are always a threat. The concept was originally designed for waste water treatment and once established, it requires minimal adjustments. The only required  maintenance is periodic replenishing of the media and cleaning of the reactor.

Determining the nitrate control methods that are best for you is all about you and your tank.

The first priority for all aquarists is prevention, ensure you are using pure RO/DI water, do not overfeed or allow leftover food to enter your filtration and, of course, maintain your filtration system.  If nitrates are continually problematic only then would you want to consider additional control methods.  I have seen reef tanks running on live rock alone and I have also witnessed tanks employing a large refugium, alongside an algae scrubber and biopellets.

Also be patient, many of these methods are not instantaneous and require time to establish and begin working to drop your nitrates.

Understanding the nitrogen cycle is key to maintaining the health of your tank

Something somewhat important to understand about biological denitrification is that the presence of phosphate is required for most of these methods to work.  Bacteria and Algae are living organisms that require carbon, nitrogen and phosphorous in order to grow and survive.  The lack of any one of these elements can limit the amount of denitrification that occurs.

For example, as macro-algae grows it consumes nitrates but also phosphate along with it. Once the available phosphate runs out, the algae cannot grow and will no longer remove nitrates. The same case applies to beneficial bacteria.  This case is rare but certainly something to consider if your having trouble establishing bacterial denitrification or nitrate fixing algae.


Chronic effects of nitrogenous compounds on survival and growth of juvenile pink shrimp. W. J. Wasieleskya, L. H. Poerscha, T. G. Martinsa and K. C. Miranda-Filhob Brazilian Journal Biology., 2017, vol. 77, no. 3, pp. 558-565

Effects of lowered pH and elevated nitrate on coral calcification. Marubini, F.; Atkinson, M. J. Biosphere 2 Center, Columbia Univ., Oracle, AZ, USA. Marine Ecology: Progress Series (1999), 188 117-121.

Nutrition of algal-invertebrate symbiosis. II. Effects of exogenous nitrogen sources on growth, photosynthesis and the rate of excretion by algal symbionts in vivo and in vitro. Taylor, D. L. Rosenstiel Sch. Mar. Atmos. Sci., Miami, FL, USA. Proceedings of the Royal Society of London, Series B: Biological Sciences (1978), 201(1145), 401-12.

F. Marubini & P. S. Davies  Nitrate increases zooxanthellae population density and reduces skeletogenesis in corals. Marine Biology (1996) 127:319 328


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