9 most important reef aquarium water parameters

9 Most Important Reef Tank Aquarium Water Parameters

The 9 MOST IMPORTANT Reef Tank Parameters are:

  1. Alkalinity
  2. Ammonia
  3. Calcium
  4. Nitrate
  5. Nitrite
  6. pH
  7. Phosphate
  8. Salinity
  9. Temperature

Let’s dive a little deeper into each individual water parameter as well as the value for each one that is considered to be ideal for a reef tank


Jump right down to the summary chart of ideal reef tank parameters

Number 1: Alkalinity

Alkalinity is a complex concept/thing to describe because it pretty much only has scientific explanations. As aquarists, we don’t care so much about the scientific definition of it, as much as we care that because of the chemistry, measuring alkalinity is a way to estimate the amount of bicarbonate that is available in the water, and bicarbonate is a very important molecular compound for the health of our reef tanks.

testing for alkalinity

Alkalinity is also sometimes called ‘hardness’, measured in dkh

Why is alkalinity important in a reef tank? Alkalinity impacts the pH of the water (pH is the number 6 most important water parameter). Equally importantly, alkalinity’s role in affecting the available bicarbonate in your tank is important, because bicarbonate is one of the main ‘ingredients’ used to build coral skeletons. You need to keep alkalinity in the appropriate range in a reef tank to ensure there is sufficient bicarbonate for your corals to grow.

Ideal Alkalinity for a Reef Tank

The ideal alkalinity for a reef tank is 8-12 dkh. That’s a pretty broad range. Perhaps more important than reaching a level within that range is to try and stay consistent with that range, once you have found an alkalinity level that suits the inhabitants of your tank and is easy to maintain, based on your local water.

Although the accepted ideal alkalinity range is 8-12 dkh, your aquarium won’t do well if the alkalinity drifts dramatically from day-to-day. Do what you can to maintain stable alkalinity–and even if you are trying to get your alkalinity up (if it is low, for example), you will want to do so very, very gradually to avoid shocking any of the animals in your system.

Number 2: Ammonia

That same chemical in your glass cleaner that helps make your glass shine and be streak-free, is probably also inside your saltwater aquarium and is the number 2 reef tank parameter to watch.

Ammonia occurs naturally, as a byproduct of some sort of organic waste breaking down in your tank. Ammonia gets into your reef tank when your fish…um…pee…and also when food or other stuff rots. From that perspective, it is perfectly natural to have it. The problem is that ammonia is toxic waste in your aquarium. In chemical terms, ammonia is a base, or a basic compound, which sort of means it is the opposite of an acid.

But while acids and bases are kind of like ‘opposites’, they cause a similar type of burning, when they contact living things. The ammonia in your tank will burn your saltwater fish and corals and even kill them, if high enough.

water parameter testing for ammonia

Ammonia test kit

The good news is that a  healthy, fully-functioning biological filter protects your tank by removing ammonia from your water. If you have detectable levels of ammonia in your tank, it means your aquarium is too new (has not fully cycled yet) or there is a problem with your biological filter.

Learn more about cycling a fish tank here.

Ideal Ammonia Parameter for a Reef Tank

As another one of the most important reef tank parameters, the ideal ammonia level for a reef aquarium is ~0 ppm. If your tank has fully cycled, there should be no detectable levels of ammonia. Ammonia can burn your fish and corals and at higher levels, it can be toxic which is why there is no tolerance for this in a reef tank and the ideal ammonia level is zero.

Number 3: Calcium

Everyone who has seen a milk commercial on television knows that calcium promotes good bone growth. And, well, they don’t drink milk. Now that I think about it, bringing up milk didn’t really help this story much. So, let’s just cut to the chase.

Calcium is an extremely important element in a reef tank and is essential for Small Polyp Stony or Large Polyp Stony Coral care and health, because, while they don’t have bones, they do have bony skeletons made from calcium, and they get that calcium from the seawater.

Calcium is the 20th element on the periodic table

Calcium might be 20th on the periodic table, but it is one of the most important reef tank parameters

For that reason, calcium is ranked as the third most important of the ideal reef tank aquarium water parameters.

Ideal reef tank calcium level

The world’s oceans tend to have reef calcium levels between 380-420 ppm (parts per million). That’s a fairly narrow range around~400, so, to keep things simple, I generally think of the optimal reef tank calcium level to be about 400 ppm.

Number 4 water parameter: Nitrate

The fourth most important water parameter for you to monitor in your reef tank is nitrate. Technically speaking, the presence of low levels of nitrates in your tank could be considered a good thing–because it means that your biological filter is working.

nitrate test

In a properly cycled aquarium, bacteria convert nitrogen waste (often in the form of ammonia) into nitrite and then other bacteria turn that nitrite into nitrate.

The problem is that as your aquarium gets more crowded and mature, it will generate more nitrate and those levels will climb.

Small amounts of nitrogen in the nitrates can be absorbed by certain soft corals or macroalgae, but it can also act as a sort of fertilizer, boosting problem algae growth–which nobody wants.

Some invertebrates are intolerant of high nitrate levels and their health will decline (adding more to the nitrates problem) if left unattended.

How to reduce nitrates from your reef tank

The fastest, most natural, and lowest-tech way to remove nitrates from your reef tank is to perform a partial water change. To help keep your nitrates low on an ongoing basis, some hobbyists employ the help of beneficial bacteria who eat the nitrates.

They accomplish this via a process called carbon dosing. What is carbon dosing, you ask?

The short version, here, is that these beneficial bacteria naturally grow in your tank, but you can give their populations a boost by adding carbon to your tank. Two common ways to do this are through the use of biopellets or vodka dosing.

Spoiler alert, the proper regimen for dosing vodka in the tank is not…one for the tank and one for me.

Ideal nitrate levels in a saltwater aquarium

It is best to keep nitrate levels as close to 0 ppm, in a saltwater aquarium. However, you may be able to ‘get away with’ slightly higher levels.

On an ongoing basis, you want to strive for nitrate levels as low as possible. However, levels around 30-40 ppm are generally tolerated by most saltwater aquarium fish (except for fragile species) and many hardy soft corals that tend to come from nutrient-rich waters. 

Number 5: Nitrite

The fifth most important saltwater aquarium water parameter is nitrite. No, that is not a typo, nitrite and nitrate are two separate and important compounds to monitor, in your water.

nitrite test kit

color-changing test kits make it easy to monitor levels of different water parameters

Nitrite is an intermediate by-product produced by your bacterial filter as part of the nitrogen cycle. In your filter, bacteria convert toxic ammonia, as a first step, into less toxic nitrite, before the second group of bacteria converts the nitrite to nitrate.

Ideal Nitrite level for a reef tank

For a very brief period of time (a few days, max), while you are cycling your tank, you will look for the presence of nitrites in your tank, as evidence that naturally occurring, beneficial bacteria have colonized your tank.

Other than that, you want your nitrites to remain as close to zero as possible. If you see any level other than zero, while you’re testing your water, it is a symptom that either your biological filter has crashed, or that it just can’t keep up with some large source of ammonia (like from a dead fish).

That is the reason that nitrite is one of the 5th most important reef tank parameters.

Number 6: Saltwater pH Level

I won’t bore you, too much, with the scientific definition of what pH is. It has something to do with hydrogen ions, the scale is logarithmic, meaning that 8.0 is a lot farther from 7.0 than it seems, and apparently, it is very important for some deodorant to be pH balanced for women (or at least that’s what the commercials tell me).

The way I think about pH is this: the pH level tells me how acidic (or not acidic) the water is.

pH test kit for reef aquarium

pH is a critical reef tank parameter to measure for a healthy saltwater aquarium

The scale reads from ‘acidic’ on the low end to ‘basic’ on the high end. The pH is essential to how all of the chemistry in your reef tank works, which is why pH is one of the 9 most important reef tank parameters.

Ideal saltwater pH level for a reef aquarium

The ideal range you want to aim for is: ~8.1-8.4 for a healthy saltwater aquarium.

While the absolute pH is important, it is perhaps even more important to ensure that the pH remains stable. Dramatic swings in pH can cause problems for your livestock.

If your pH either starts or falls out of that range, take your time raising or lowering the water over hours or days (depending on how dramatic the difference is) to avoid shocking your fish, corals and other invertebrates.

Number 7: Phosphate

The seventh most important reef tank water parameter to monitor is phosphate.

Phosphate occurs naturally on reefs and is present at a level of ~0.13 ppm, but phosphates in a saltwater aquarium can be a big-time problem-maker in your tank, even at natural levels.

Because in your saltwater aquarium, phosphate acts as a fertilizer for algae, and if your tank is like mine, you probably don’t have the same types of algae-eating controls as a natural reef does.

Ideal phosphate level for a saltwater tank

Determining the ideal level of phosphates for a reef tank is a bit of a tricky subject. Since it fuels problem algae growth, you want to keep it low and may be tempted to keep it at or near zero.

But it is also a nutrient.

Because of that, I recommend you keep levels below 0.2 ppm if possible.<0.2 ppm.

To keep your levels low, you might consider running a phosphate reducing media in a media reactor.

Number 8: Salinity

The eighth most important reef tank water parameter is salinity. When I say that the salinity of the reef tank water is important, what I’m referring to is the amount of salt in the salt water. Get it?

But salinity is relatively difficult to measure directly.

One way you could measure the salinity of the water is to completely evaporate a liter of saltwater and weigh the salt that is left behind. The salt left behind from a liter of saltwater would be your salinity.

Luckily, some super-smart people came up with a few super-easy ways to estimate salinity super-easily.

When the salinity of the water changes, two other important things change:

  • the density of the water changes and
  • the way it bends light changes

So, as hobbyists, we use a hydrometer to measure the density (specific gravity) of the water, or we use a refractometer to measure the angle of light refraction, to estimate the salinity.

Ok, speaking of bending light, I love this band:


You have to have salt in the water to measure salinity. Most of us make our own seawater with a salt mix.

Check out this article to learn about the best reef salt mix for a marine aquarium. The article includes a comparison chart, lists the various parameters and provides some information to compare the relative value-for-the-money for each product.

Ideal reef tank salinity level–measured as specific gravity

The salinity of the ocean is actually ~ 35 g/L, but for your saltwater aquarium, it is more common to measure the specific gravity of the water as a proxy for salinity, because of how easily specific gravity can be measured. Ideally, you want to keep your aquarium reef at a specific gravity of  1.025, which is the simplest way to ensure the salinity replicates the salt concentration of a natural reef.

refractometer is sometimes used to measure salinity of reef tank water

The amount of salt in reef tank water bends the light in a predictable way allowing us to measure salinity

Number 9: Temperature

The ninth most important reef tank water parameter is temperature.

The majority of animals we keep in our tanks come from tropical reefs, and tropical reefs tend to have a stable temperature somewhere between 73 and 84 degrees Fahrenheit, or 22 to 29 degrees Celcius.

If you live in an area where the ambient temperature matches that range…

well, then I’m a bit jealous and hope you’ll invite me over to hang out very soon.

Thermometer noting typical range

Maintaining your water temperature in the right range is vital for reef tank success

If you don’t live in an area with a steady tropical temperature like that, then you will need equipment to keep the aquarium temperature in the right zone.

An aquarium heater is probably required for just about any aquarium because most of us live in houses where the daily temperature is lower than the ideal values listed below. The aquarium heater raises the water temperature and helps keep it from falling below that level.

If you live in a warm climate, where the temperature gets above the ideal range, you may need an aquarium chiller. Check out this article for reviews to find the best aquarium chiller for your aquarium.

Ideal Value

Since most of what we do is intended to recreate the natural environment our animal friends have come from, it should be no surprise to you that the ideal recommended temperature for your reef tank is the same. The recommended range is:

73-84 Fahrenheit or 22-29 Celcius

That’s a pretty broad range–and you might experience some challenges with finicky species at the extremes. Within that acceptable range, the most commonly provided advice tends to be to keep your aquarium temperature around 78-80 degrees Fahrenheit or 25.5-26.6 degrees Celcius.

As long as the temperature of your saltwater aquarium is in this range, keeping the temperature consistent (avoiding fluctuation) becomes more important than the actual value itself.

A chart of the 9 Most Important Reef Tank Parameters and the Ideal Reef Tank Parameter Values


Want to take a copy of this information with you?

Get a FREE downloadable reef aquarium parameters spreadsheet you can keep on your phone or desktop


reef tank parameters chart free for downloading and printing

Free downloadable reef tank parameters chart

Other important reef aquarium parameters (not in the Top 9)

Before you go, please take a moment to familiarize yourself with three other important reef tank parameters that are important, just not one of the 9 MOST IMPORTANT.

Those three items to monitor are:

Other important water parameters

There are a couple of reasons those three reef tank parameters didn’t make the cut.

The first is that, well, the list of the 9 most important items stops at 9. You have to draw the line somewhere, and I did my best to prioritize the top 9, based on my own knowledge and experiences.

Additionally, these next 3 parameters are not as practical to measure or moderate through supplementation and water changes as the major items in the top 9.

Please don’t read this as me saying that these three elements are not important to reef coral biology, because they are.

The critical factor is not that these are irrelevant biologically, but that they are simply not as practical for the average hobbyist to focus on.

As best I can tell, the science supporting the dosing of these trace elements in a reef aquarium is inconclusive. So, I took them off the list. No sense measuring something you don’t intend to act upon.


Iodine, as a trace element does appear to be important to several macroalgae, shrimp, and coral species, but because natural levels are so low (0.06 ppm), it is very difficult to test and maintain these levels with standard test kits. As such, I don’t recommend dosing iodine as a supplement with the intent to keep levels consistent with natural seawater.

iodine is the 53rd element on the periodic table

Iodine is also an important element for reef tanks

For a while, it was speculated that iodine was a necessary element to add in order to keep pulsing xenia alive, but later that information was refuted.

Observational and sometimes speculative information like that is what makes it challenging as a hobbyist, sometimes, to know what to focus on and avoid doing more harm than good, in the process of trying to create the ideal living conditions.

Ideal Value

The ideal concentration of iodine in a reef tank is 0.06 ppm. Because those levels are so low, because testing would be so complex and because the benefit of maintaining that precisely is unknown while the risks of adding too much (toxicity) are quite known, it is probably best to resist the urge to monitor and adjust your iodine levels manually. Instead, buy a high-quality salt mix that roughly approximates the natural levels of the ocean and stay on top of your water changes.


Magnesium is an important ion and is the third most abundant ion in seawater. For the advanced reefer, it can play a major role in creating some of the problems advanced reefers face–however, for the vast majority of us, this tremendously important ion is naturally present in such large amounts that it renders it as important, but probably not worth worrying about on an ongoing basis.


As such, I put it in the ‘nice to know, but don’t need to worry’ bucket. At least for right now.

If you’re already an advanced aquarist, then, you may want to dig a little deeper there. More information can be found here.

Ideal Value

The best magnesium level for a reef tank is likely consistent with the range found in the ocean–approximately 1285-1300 ppm. Levels that drift significantly outside of that range can cause problems, including causing issues with other parameters like calcium levels in your tank. But until you reach an advanced level of sophistication or if you’re otherwise experiencing problems with your water quality and need to rule it out as part of the problem, you may be able to let sleeping magnesium lie and focus on the 9 most important water parameters.


Strontium is actually a bit of a controversial supplement in the saltwater aquarium hobby (well, I guess as controversial as something like strontium supplementation could be).

I just let those words wash over me for a moment and pictured a hotly debated strontium controversy between two uber fish geeks. Okay, back to the article…

If you want to learn more about Strontium than most chemists (slight exaggeration there) check out this article. By the way, the author states that typical ocean levels of strontium are 8 ppm.

Ideal Value

~8 ppm


So those are the 9 MOST IMPORTANT marine aquarium water parameters plus 3 more important items that are important, but just not worth a lot of your time and effort (in most cases) unless you are having serious problems and are convinced the top 9 values are fine. Ok, so now you know where to focus your attention, in the beginning, but what are you supposed to do about it?

Test your reef tank parameters

You need to test your aquarium water to be sure it is in a healthy and suitable range. The next several links to test kits are affiliate links that will take you to the Amazon.com product page for those products, where you can review the specifications more closely (if you wish) and read user reviews to decide for yourself.

Testing your reef tank water is essential

Just so you know, I do earn a tiny commission if you purchase anything on Amazon after visiting through one of those links. No pressure to do so, just letting you know those are affiliate links. To tackle the big four reef tank aquarium water tests (ammonia, nitrite, nitrate, and pH), you may want to check out the API Saltwater Master Test Kit. Using this kit, you can test for the four big aquarium water parameters in just a few minutes.

You can find this kit in most of the major fish stores (including the big chains). I was surprised to see how expensive it was at retail compared with online. This is the kit I used when I set up my first aquarium, and I have replaced it more than once. For alkalinity, phosphate, and calcium, I have generally used individual test kits

Equipment for Measuring Reef Tank Parameters

For equipment, I now use this Refractometer to measure the salinity of my reef tank, although you could certainly use a swing-arm hydrometer, like the one shown in the image above. They are inexpensive and easy-to-use.

For temperature, I use this Digital Aquarium Thermometer that I bought, a while back, on Amazon. Now, a few thoughts here about reef tank aquarium water testing: no test kit, intended for hobby use, is going to be perfect. Test kits can sometimes get a bad reputation, or get blasted in online forums for their unreliability or lack of precision.

Equipment for Maintaining Aquarium Water Parameters

High-quality reef tank water starts by using a high-quality reef aquarium salt mix. To keep your water parameters in the ideal range, you may want to have a protein skimmer, installed in a sump with a refugium, a calcium reactor, and reliable heaters and/or a chiller.

Biological filtration to keep your water pure

In a marine aquarium, the biological filter will help you keep your water in tip-top shape. Helpful bacteria that live on and in the live rock, live sand, and even on the aquarium glass itself will help remove toxic ammonia from the water and replace it with much less toxic nitrite. Then, the second group of helpful bacteria will convert that nitrite into nitrate.

If you choose to add macroalgae to your sump or even to the display tank itself, the macro will actually pull phosphates, silicates, and nitrates out of the water as well.

If you want to take this biological filtration one step further, you can add a deep sand bed to your tank or refugium. The deep sand bed creates a low-oxygen environment at the bottom of the bed where the third type of beneficial bacteria can grow.

These natural, biological methods of keeping the water clean will help you keep the water in your home reef in a range that is suitable for growing the most amazing corals.

9 most important reef tank parameters


  • Regardless of whether you spend $7 on an API test or ~$20 for a Salifert test kit, you need to take some measures to ensure your glassware is clean (and not contaminated) and that you perform the test according to the included instructions. If not, you could get false readings on your reef tank parameters.
  • Keep clean glassware. If not, you can’t rely on the test results
  • There is no substitute for observing your reef tank and visually determining if the data point from your test kit is consistent with what your eyes see. Test kits can and do fail.
  • If you get a really high nitrate reading but your tank is telling you otherwise…do a water change (just to be safe) but get another kit to verify it isn’t a false reading.
  • In order to help keep your water quality as high as possible, you may want to purchase a high-quality protein skimmer. Check out this article for a review of the best protein skimmer models.
  • If water quality isn’t your problem, but you have some sort of pest, parasite, or infection, you may need to move your sick animals to a hospital tank and treat them with an aquarium medication, instead. You can learn more about those here.


A big disclaimer that is appropriate for this entire article is that what I’ve listed above are general reef tank parameters.

It is entirely possible that specific individual species you acquire for your own tank may come from an environment that differs from these generalities and may, therefore, require specific care. It is up to you to research the husbandry needs of the animals you want to keep to be sure you know if their needs differ from the standard water parameters.

And if your animals require specific aquarium water parameters–you should do your best to meet those standards–or steer clear of those animals.

Additional reading and information

Thanks for checking out this article about the most important reef tank parameters.

Continue the journey to read, learn, and explore by checking out these books.

If you want to keep reading and learn some more about the most important reef tank aquarium water parameters, or other topics related to keeping a successful saltwater tank, check out the links below to some helpful articles:

To continue learning about this topic, check out this video:

A Beginners Guide to Reef Tank Water Parameters

The inspiration for this ideal reef tank parameters article

This article about the most important reef tank parameters was inspired by a short blurb in an old Doctors Foster and Smith catalog, titled “Ideal Marine Aquarium Water Parameters: Key to Continued Inhabitant Health”. The article was short, to the point, and communicated a lot of information in a single catalog page.

The aquarium water parameters listed on this page are adapted from  Wikipedia, Dr. Foster and Smith, and Advanced Aquarist. I cross-checked the information in their table with other reputable sources. But it also created a few unanswered questions in my mind. Questions like: which parameters are the most important? And how close to ‘ideal’ do you need to keep the water parameters in your own aquarium without causing problems?

Hopefully, with this piece, I managed to cover not just what the ideal reef tank water parameters are, but also which of the reef tank parameters are most important.

What do you think–are there any saltwater parameters more important than these 9? If so, please leave a comment below.

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