9 most important reef aquarium water parameters

9 Most Important Reef Tank Aquarium Water Parameters

Albert B Ulrich III aquarium water parameters 4 Comments

I was flipping through a copy of the Doctors Foster and Smith catalog, and was inspired by a simple, yet helpful article titled “Ideal Marine Aquarium Water Parameters: Key to Continued Inhabitant Health”. The article was short, simple and communicated a lot of information in single catalog page–but I found it also created a lot of follow-up questions in my mind. Questions like: which parameters are the most important? And how close to ‘ideal’ do you need keep the water parameters in your own aquarium without causing problems? With that original article as inspiration, here are the 9 most important reef tank aquarium water parameters.

I cross-checked the information in their table with other reputable sources. The following suggested aquarium water parameters are adapted from LiveAquaria, Wikipedia, Fishchannel.com and Advanced Aquarist:

Here are the 9 MOST IMPORTANT Reef Aquarium Water Parameters

  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 one:

9 most important reef aquarium water parameters


Alkalinity is a complex concept/thing to contemplate. As aquarists, we don’t care so much about the scientific definition of it, as much as we care that it is a proxy (a way to estimate) the amount of bicarbonate available in the water–because bicarbonate is essential for coral health.

Ideal Value

8-12 dkh


Ammonia is a toxic waste in your aquarium. Except for when you are cycling your tank, you want ammonia levels to be as close to zero as possible.

Ideal Value

~0 ppm


Calcium is another essential element for coral health in a saltwater aquarium. According to the Drs. Foster and Smith chart, natural coral reefs tend to have caclium levels between 380-420 ppm (parts per million). For simplicity sake, I find 400 ppm to be a suitable approximate value. Calcium is extremely important for LPS Coral and SPS Coral.

Ideal value

~0 ppm


In a properly cycled aquarium, the presence of nitrate is confirmation that your biological filter is working. Congratulations on that. 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 soft corals that tend to come from nutrient rich waters.

Ideal value

~0 ppm


Nitrite is an intermediate by-product produced by your bacterial filter. In your filter, bacteria convert toxic ammonia into less toxic nitrite and then nitrite is further converted into an even more safe chemical called nitrate. Except when cycling your tank, nitrite levels should remain as close to zero as possible.

Ideal Value

~0 ppm


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 live stock.

Ideal Value



On natural reefs, phosphate is present at a level of ~0.13 ppm. In your saltwater aquarium, it acts as a fertilizer for algae–because of that, I recommend you keep levels below 0.2 ppm if possible.

Ideal Value

<0.2 ppm


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. If your zoanthids have closed up, check your salinity.

Ideal Value

Measured as specific gravity: 1.025


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. I have most commonly seen/heard recommended temperatures around 78 degrees Fahrenheit (25.5 degrees Celsius).

Ideal Value

73-84 Fahrenheit or 32-29 Celcius

If you are interested, you can view a printable version of these aquarium water parameters:


Other important water parameters (not in the Top 9)

There are three other aquarium water parameters on the Drs. Foster and Smith chart that I left off of my list of the 9 MOST IMPORTANT reef tank aquarium water parameters:

  • iodine
  • magnesium
  • strontium

Other important water parameters

The reason those three aquarium water parameters didn’t make the cut is that they are not practical to measure or dose in a saltwater aquarium. Don’t read this the wrong way–all three are important to reef coral biology. The critical factor is not that these are irrelevant biologically, but that they are not practical for the casual hobbyist. Magnesium is a tremendously important ion–but it is available in such large amounts in a typical aquarium that it is all but irrelevant for most aquariums. Iodine and Strontium, on the other hand, are important trace elements–but their concentrations are generally so low that it is not practical to dose them, measure them or otherwise deal with them in any reasonable fashion. 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 macro algae, 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.

Ideal Value

o.06 ppm


Magnesium is the third-most abundant ion in seawater. It is an extremely important ion, but since it is generally present in such high quantities, measuring it and worrying about it just doesn’t seem that practical to me. As such, I put it in the ‘nice to know, but don’t need to worry’ bucket.

Ideal Value

1285-1300 ppm


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). 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 reef tank aquarium water parameters and 3 important aquarium water parameters that are just not worth your time and effort (in most cases). But now that you know what the most important water parameters are, what are you supposed to do about it?

Testing your water

saltwater aquarium water parameters testing Now that you know which water parameters are the most important to pay attention to, you should be sure to test your aquarium water to be sure your aquarium water is in a 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. Just so you know, I do earn a tiny commission if you purchase anything on Amazon after you visit 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 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


For equipment, I now use this Refractometer  to measure salinity. 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.


Regardless of whether you spend $7 on an API test or $25 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–and even then, treat each test as a single data point. If you suspect a problem, 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. Because 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. Also don’t make the mistake of thinking that the hobby test you bought for a few dollars is as reliable as reagents or equipment that would cost an analytical lab a few hundred dollars. What’s most important, when testing at home, is detecting changes in water parameters as they are happening, using that information to find the root cause and fixing it.

At home test kits are not for writing your thesis paper or defending the absolute value of the water parameter in question. You could spend a whole lot more money on more expensive test kits if that suits you, your budget or your approach to testing. For me, close enough is good enough–and I’m comfortable knowing that there are so many other factors affecting the test results (like my own sloppiness, lack of technique, etc.) that a good-old mass-market test is good enough for me. But I encourage you to decide for yourself.

A big disclaimer that is appropriate for this entire article (a version of a similar disclaimer is also on the LiveAquaria website) is that what I’ve listed above are general aquarium water 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

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, check out the links below to some helpful articles:

Written by Albert B. Ulrich III. Follow me on Google + and Twitter

Albert B Ulrich III9 Most Important Reef Tank Aquarium Water Parameters

Comments 4

  1. roger howard

    in my case, i found Magnesium to be of HIGH importance.

    my calcium levels were very low at 340ppm and it appears that daily dosing of calcium supplements did not help it raise one bit. same issue with my alkalinity, it wouldn’t budge from a low 5.

    turns out my magnesium was low at 1220 or so and it was affecting my tanks ability to raise the calcium and alkalinity levels .

    as soon as i started dosing magnesium, my calcium and alkalinity supplements were finally helping me with reach levels closer to normal.

  2. Post
    Albert B Ulrich III

    Hi Roger,

    Thanks for the comment. It is a helpful reminder that generalities are generally wrong. Sorry to hear you were having Mg and Ca level issues but glad to hear your tank is back. How much Mg do you have to add? The author of this article said they needed 2 pounds of the stuff to raise 200ppm. Do you get it up to 1250-1285 or do you push even higher? What were the problems you were seeing in your tank as a result of the low levels? What good things are you seeing now that the levels are higher? Any other advice for someone facing the same issue? Thanks for your help filling in a gap here and for the thoughtful comment.

  3. Jeremy

    I don’t know why anyone would tell anyone mag is least important, If you are trying to keep corals and calcium and alkalinity stable. You have to have mag.mag keeps the ions of calcium and alkalinity from locking together. If your mag is not correct you will find problems trying to stable out your perimeters. .don’t believe everything you read on the net .. this article has misinformed ppl

  4. Post
    Albert B Ulrich III

    Hi Jeremy, thanks for the comment. While I feel like the tone may be a bit harsh (maybe I’m taking it too personally), it does provide an opportunity to explore your point and help ensure there is good information out there .

    Here are a few quotes in the piece to highlight, that articulate the position about magnesium:
    “The reason those three aquarium water parameters (iodine, magnesium, strontium) didn’t make the cut is that they are not practical to measure or dose in a saltwater aquarium. Don’t read this the wrong way–all three are important to reef coral biology.”

    “The critical factor is not that these are irrelevant biologically, but that they are not practical for the casual hobbyist. ”

    The article also states:
    “Magnesium is a tremendously important ion–but it is available in such large amounts in a typical aquarium that it is all but irrelevant” –Commentary (I should have added…to measure and dose, unless you believe there is a magnesium problem) for most aquariums.

    In the table, the listing for magnesium states:
    “Magnesium is the third-most abundant ion in seawater. It is an extremely important ion, but since it is generally present in such high quantities, measuring it and worrying about it just doesn’t seem that practical to me. As such, I put it in the ‘nice to know, but don’t need to worry’ bucket”

    What I’ve found in talking with people who are just starting out with the hobby is that there is so much expense, so many kits, so much equipment and so many things to track and monitor that it can be intimidating or paralyzing. The position I took in this article is to suggest that for the beginner, you should focus first on the top 9 and in the absence of symptoms of a magnesium problem…magnesium, being such a large part of salt mixes naturally… you can afford not to measure, in the same way we don’t measure sodium or chloride in the water…they are abundant, in most cases and are naturally replaced with water changes. I hope that helps clarify, a little, and seeks to be consistent with your charge of providing good information.

    There is also a link out to an article that specifically provides more information about magnesium, for those who want to dive deeper. That article, by Dr. Holmes-Farley, which covers magnesium in significantly more depth than what is covered in my simple overview, has the following information in it:
    “The primary reason that magnesium is not more of a daily concern to aquarists is that the reservoir of magnesium in seawater is very large…Consequently, maintenance of magnesium levels is not typically a rapidly developing problem. If using an appropriate salt mix, it may never become a problem for many aquarists.”
    Please let me know what other clarifications you think would be helpful to add related to magnesium or any of the other water parameters on this page. I appreciate the feedback and your passion about making sure the advice is helpful and accurate. I’d also be interested to know what water parameter you’d take out of the top 9 (in terms of testing and monitoring) to replace with magnesium, that might be a helpful exercise too, to hear how you would prioritize them and provide a counter-point to the prioritization method used here.

    Thanks again for your comment.

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