Reef Tank Maintenance

9 Most Important Reef Tank Aquarium Water Parameters

Here are the 9 MOST IMPORTANT Reef Tank 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 individual water parameter as well as the value considered to be ideal for a reef tank:

1. Alkalinity

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–it is one of the main ‘ingredients’ used to build coral skeletons.

Even though it is difficult to understand, as a concept, rest assured that alkalinity is one of the most important reef tank parameters.

Ideal Alkalinity for a Reef Tank

The ideal alkalinity for a reef tank is 8-12 dkh. That’s a pretty broad range. The goal, as with most of these water parameters, is to maintain consistency. Even though the accepted range is 8-12 dkh, it doesn’t mean your tank will 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.

2. Ammonia

Ammonia is 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. Ammonia gets into your reef tank when your fish…um…pee…and also when food or other stuff rots.

A healthy, fully-functioning biological filter will remove 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.

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.

3. Calcium

Calcium is another essential element for coral health in a saltwater tank and is one of the most important reef aquarium parameters. According to the Drs. Foster and Smith chart, natural coral reefs tend to have calcium 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 reef tank parameter for Calcium

The ideal marine aquarium water parameter for Calcium is ~400 ppm

4. Nitrate

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.

In order to help keep your nitrates as low as possible, you may want to purchase a high-quality protein skimmer.

Ideal reef aquarium value for Nitrate

The ideal reef aquarium value for nitrates is ~0 ppm. However, as mentioned above, you may be able to ‘get away with’ slightly higher levels.

5. Nitrite

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.

That is the reason that nitrite is one of the 9 most important reef tank parameters.

Ideal Nitrite level for a reef tank

~0 ppm

6. pH

I won’t bore you, too much, with the scientific definition of what pH is. The simple version is that it has to do with how acidic (or not acidic) the water is.

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.

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.

Ideal pH for a reef aquarium

~8.1 – 8.4

7. Phosphate

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 phosphate level for a saltwater tank

<0.2 ppm

8. Salinity

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.

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

Ideal reef tank salinity level–measured as specific gravity

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.

9. Temperature

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).

To keep your aquarium temperature stable, you might need two pieces of equipment.

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.

Ideal Value

73-84 Fahrenheit or 22-29 Celcius

Other important reef aquarium parameters

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

  1. Iodine
  2. Magnesium
  3. Strontium

The reason those three reef tank 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 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.

Ideal Value

0.06 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. It is a critically important reef tank parameter, but it tends to only be a problem in advanced situations.

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.

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).

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, at 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 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 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 macro algae 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 bacteria actually remove nitrates from the water, as well.

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.


  • 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.
  • 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 with an aquarium medication, instead.
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