Keeping nitrate under control has always been a concern with reef aquarists because nitrate is the end-product of the nitrogen cycle. Nitrification via your biological filter is continually creating nitrates as fish waste and food break down. In fact, nitrate is one of the primary reasons we filter the aquarium. No matter what kind of tank you choose to have, fresh or salt, SPS or Fish-Only - nitrates play a key role in the overall health and are critical to the biological foundation of the tank.

The conversation around nitrates has evolved over the years and so has the way we manage them. Keeping nitrates at safe and healthy levels is the classic struggle for aquarists but this struggle is changing as we begin to understand more about nitrates, how they impact the tank, and discover new and better ways to manage them.

So whether you are constantly battling elevated nitrates or grasping to hold onto every single nitrogen molecule you can, here’s what you need to know to take control of nitrates in your reef tank!

Nitrogen cycle Understanding the nitrogen cycle is key to maintaining nitrates.

What Is Nitrate & Why It Matters

Nitrate is the end by-product of the nitrogen cycle. As fish food, waste, and other organic matter are broken down by bacteria in your tank, nitrate is ultimately produced. Some of this nitrate can be processed further and turned into nitrogen gas and released into the atmosphere, but a majority of it just stacks up in the tank water. 

Nitrate exists in two forms in your tank, organic nitrate which is nitrogen that exists inside organic matter (fish food, fish waste, algae, bacteria, etc), and then dissolved nitrate in the aquarium water which is what we test for. Organic nitrogen is essentially the source of the dissolved nitrates that you are testing or monitoring in the water.    

Dissolved nitrates themselves are not directly toxic to your tank inhabitants but at elevated levels, they can start to be problematic. Nearly all the scientific research about the effects of nitrate in an aquarium has been conducted with freshwater fish and invertebrates. Naturally, these results can’t be applied to saltwater fish and invertebrates, specifically corals.

What little information is available seems to indicate nitrate is not all that harmful, at least when compared to toxic ammonia and nitrite. We can confirm this from anecdotal evidence in our tanks; there have been successful reef tanks with nitrates as high as 30+ ppm as well as successful tanks with undetectable levels of nitrate <1.0 ppm. That said, there are pretty clear reasons hobbyists continually monitor nitrates and keep them under control. 

Nitrates's effect on corals

As it pertains to corals, we found a study performed with reef-building corals, Porities and Montastrea, that demonstrated 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. Most of the science and anecdotal evidence shows nitrate is not a deadly killer of fish, corals, or invertebrates. With that in mind, hobbyists are still striving to always manage nitrates at reasonable levels. WHY?

Nitrogen is required by all living organisms, starving the tank of nitrogen altogether can be deadly therefore running too clean or devoid of nitrate altogether can lead to serious problems as proven in recent years with strong filtration methods, refugiums, and carbon dosing. Elevated nitrates can lead to nuisance algae and other pests, bacterial imbalances, inhibit coral growth and, even stress out aquarium inhabitants.  

So the question becomes what is the ideal level of nitrates for a reef tank? We need some nitrate present, but how much is too much?

Before we get into that, let's talk about nuisance algae and using nitrate as a general water quality indicator which is perhaps a more critical reason to monitor them along with phosphates.  

Siphoning algae off of rock Excess Nitrate and Phosphate levels can contribute to algae growth

Excessive nitrates will most certainly contribute to nuisance algae growth.  If coral and bacteria are not taking up the dissolved nitrates in the water, algae can most certainly fill that niche. Not to say high nitrates always result in an algae problem, you are just much more likely to get into algae trouble faster when your providing nutrients via dissolved nitrate.

Nitrate is very useful for hobbyists as a general indicator of water quality.  As nitrates rise, so do phosphates and other organic compounds that can stain your water and even cause odors. For the most part, you're going to see phosphate rise alongside nitrate in an aquarium. Where there is high nitrate, you're often going to find elevated phosphate and phosphate comes with a very similar set of consequences in a fish tank. As a hobbyist, we can use nitrate as a good indicator of when to change the water and gauge our filtration and maintenance routine.  

Aquarists are beginning to understand that keeping the ideal ratio of nitrate to phosphate level in your tank will support a healthy and diverse bacterial population that outcompetes things like algae, cyanobacteria, and dinoflagellates. As we start to learn more about the microbiome in our tanks and catalog what that looks like, we learn more about the necessary nutrients required to maintain that healthy biome.

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What Is The Recommend Nitrate Level

The recommended nitrate levels in a reef tank can be confusing, you will likely get different answers based on who you talk to or what you read. This information has most certainly changed over the years and will continue to evolve. The facts are nitrate levels on a wild reef in the ocean are below 0.1 ppm. If we make this the benchmark, nitrate levels should always measure zero using home aquarium test kits but there really is more to the story.  

Our captive reef tanks are drastically different than wild reefs. Many successful reef aquarists keep beautiful, thriving fish and corals with nitrate levels above 10 ppm. There are also plenty of tanks with much lower nitrates that are thriving. Keeping nitrates in the 1-5 ppm range in a soft coral or LPS tank is considered acceptable whereas SPS corals tend to grow better in Ultra Low Nutrient tanks with levels below 1.0 ppm. 

The general idea for reef tanks is to keep nitrates under control, without stressing over a specific level rather not letting them swing or get extraordinarily high at any given time. If your filtration is working, you're feeding correctly with an appropriate bioload and you're maintaining the tank correctly with water changes, elevated nitrates will not be a threat.

Keeping the nitrate under 10 ppm seems to be the norm and for those attempting to maintain an ultra-low nutrient tank, below 1.0 ppm is the goal.

The Redfield Ratio is something that has dominated this discussion in recent years which is simply the natural ratio of carbon:nitrate:phosphate found in our oceans.  The thought is if we can mimic this ratio in our tanks, the goal is achieved and we can maintain a healthy microbiome and ensure our corals are getting the nutrition they require. After all, they evolved to eat prey with this particular ratio of nutrients. In this case, it is less about the exact number and more about the 106 carbon: 16 nitrogen: 1 phosphorous ratio. So 16ppm nitrate in your tank would call for phosphate at 1ppm to maintain a healthy environment.

This is yet to be proven with hard data in aquariums but anecdotal evidence makes a strong case. Check Ryan's BRStv Episode: A Magic Ratio? Trailblazing Pest Solutions. Nitrate & Phosphate Ratios vs Concentration for a more in-depth discussion.

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How To Measure Nitrates?

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. ICP testing or mail-in water test kits are also a great choice but it does take more time to get the results. The new Nitrate Checkers by Hanna Instruments are becoming quite the popular option now as well. 

We recommend testing for nitrates every 1-2 weeks or in between water changes to monitor nutrient levels.  Test nitrate and phosphates at the same time to be sure they are both being maintained. Consider printing our Reef Aquarium Parameters chart to keep as a handy reference.  

How To Control Nitrates In A Reef Tank

Water Changes

Contrary to popular belief, a typical water change is great for diluting organics or re-balancing elements but doesn't work so well for controlling nitrates directly. The typical 10% - 20% water change, even if made every week, will never be able to significantly decrease nitrates once they have been elevated. 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.

That said, you can cut back on food significantly and perform larger 50% or more water changes every few days to lower nitrates. This is really not a reasonable approach to maintaining low levels of nitrate day in and day out. Especially on larger tanks where 50% of the volume is 50 gallons of water or more. This approach can also stress the tank out with drastic water chemistry changes and is only recommended when drastic measures must be taken.  

RO/DI System

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 contain 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.

Bioload and Feeding

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 are fun, but too much will certainly get you into trouble fast. So mind your bioload, feed responsibly using high-quality foods, turn off your filtration during feeding, and don't overfeed. Practice this from the very first fish you add to the tank and forevermore, creating good habits.

Ignoring their bioload is probably the #1 reason aquarists run into trouble with nitrates, simply adding too many fish and/or too much food for the tank and its filtration to handle.

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

Biological Denitrification

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 some reef tanks, this is all that’s needed to keep nitrate levels low alongside an appropriate fish stocking level, mechanical filtration, and maintenance. This was especially true in the days of real live rock pulled out of the ocean which is much more porous compared to the dry rock we are using in reef tanks today.  

Deep sand beds create anaerobic environments too which help reduce nitrates, but this is a somewhat dated technique we see less and less in modern aquaria. They get dirty over time and are hard to keep clean and when organic matter finds it way deep into the sand, dangerous hydrogen sulfide can be formed. There are just better ways to accomplish the same nitrate reducing result.

Special biological filter media can also be used to create an anaerobic environment and harbor denitrifying bacteria.  Seachem Denitrate and Brightwell Export NO3 are both popular options.

A sulfur denitrator is a special filter that relies on biological denitrification. A recirculating reactor that houses a bed of sulfur media below a bed of calcium carbonate media is employed. An anaerobic colony of nitrate-consuming bacteria is established on the sulfur media which ultimately turn nitrate into nitrogen gas, effectively removing nitrates from the aquarium.

There have also been a handful of other nitrate filters designed by/for aquarists over the years including alcohol-based reactors and slow flow nitrate coils, both of which use bacteria to get the job done. It is safe to say sulfur denitrators or bio-denitrator-type reactors, in general, are not all that common these days. With modern filtration techniques, including refugiums and algae scrubbers, these nitrate filters have become almost obsolete.

We could also lump carbon dosing and biopellets into this "biofiltration" category since bacteria is doing the work, but at the same time there is a major difference. With biopellts and carbon, you are not harnessing "anaerobic bacteria" rather "aerobic bacteria" to uptake nitrate.

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Carbon Dosing

In cases in which nitrates are consistently a problem or when hobbyists wish to keep ULNS - Ultra Low Nutrient conditions(<0.05 ppm PO4 and <1 ppm NO3) in the tank. Nitrate reducing additives, in their many formulations, provide carbon (often called "carbon dosing") that fuels the growth of beneficial bacteria at accelerated rates, resulting in the removal of both phosphate and nitrate from the aquarium water. Most carbon dosing routines call for a protein skimmer to ensure dissolved oxygen levels are present and help export some of the nitrate-fixing bacteria from the aquarium.

Carbon dosing is quite popular in reef tanks because the process also creates an ideal food source (bacteria) for the corals.  Red Sea Nopox, Tropic Marine NP Bacto-Balance, and Brightwell Aquatics Reef BioFuel are popular choices and hobbyists have also created a few DIY carbon dosing recipes including the popular "Vodka Dosing".

Biopellets

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 substrate for the bacteria to colonize on. The pellets are tumbled heavily to constantly sluff the bacteria growth or "biofilm" which then exits the reactor and is removed via a protein skimmer or other mechanical filtration. This process is very similar to carbon dosing but is done inside a reactor. Proper water flow through the reactor is critical and most products recommend a very efficient protein skimmer.

Refugiums

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 house macroalgae with the appropriate lighting to support it. Macroalgae use nitrogen for growth and therefore when grown in a refugium, nitrate will be removed from the water as the algae grow. When the algae are harvested from the refugium, the nitrate comes with it. This same principle applies to an algae reactor such as the Pax Bellum units, macroalgae grow inside the reactor consuming nitrates as it grows.

Refugiums come in many forms and can be built right into your sump, isolated inside a small chamber in your display, in the back of your AIO tank, or even hanging off the side of your display. They are highly effective in terms of nutrient control when maintained properly and also provide the benefit of increasing biodiversity in the tank.  Hobbyists will harbor microorganisms such as copepods and amphipods in the refugium which gives these tasty creatures a safe place to reproduce while still feeding the animals in your display.  

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Algae Scrubbers

Algae scrubbers operate on a similar concept to refugium 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 just like macroalgae does. Then just periodically harvest the algae to physically remove the nitrogen from the aquarium.

Again, a very effective and popular option for modern reef tank owners.  Algae don't require constant replenishment with expensive media, it doesn't require a ton of maintenance and can effectively keep up with typical nutrient levels in a reef tank. 

Mechanical Filtration

Mechanical filtration is often sort of skipped as a nitrate control method but shouldn't be.  Mechanical filtration is your filter socks, filter pads, filter sponges, and protein skimmer. Anything that physically removes organic matter from the tank.  Since this organic matter is the source of your nitrates, you can effectively remove the waste before it becomes a nitrate problem.  Effective mechanical filtration will ultimately result in lower nitrate levels in your tank.

SOURCES:

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