9 most important reef aquarium water parameters

9 Most Important Reef Tank Parameters

Everyone loves adding fish, coral, and other invertebrates to their reef tanks. But before you can jump into that fun, you need to make sure you have your water on point. Reef tank water parameters are crucial for the health and well-being of EVERYTHING you add to the aquarium. And it can feel overwhelming. But armed with all of the information, you’ll find yourself testing and regulating your water conditions like a pro.

We’ll kick things off with the nine MOST IMPORTANT REEF TANK PARAMETERS before moving on to a few additional key components. And then, we can “dive into” how to test those conditions and manage tricky situations. If you want to focus on a particular water parameter, feel free to use the links below. Otherwise, you can read through the entire article and make sure you’re getting the hang of appropriate reef tank water conditions.

In a hurry? No problem: you can find handy charts of the top nine ideal reef tank parameters AND the other three reef aquarium water parameters!

1: Alkalinity

Alkalinity is a complex concept to describe because it revolves around scientific explanations. As aquarists, we don’t care about the scientific definitions. We care about chemistry. Measuring alkalinity is a way to estimate the amount of bicarbonate available in the water. Bicarbonate is a vital molecular compound for the health of our reef tanks.

testing for alkalinity

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

Why is alkalinity important in a reef tank? Alkalinity impacts the pH of the water (pH comes in as the sixth most important water parameter). Alkalinity’s role in affecting the available bicarbonate in your tank is also important because bicarb is one of the main ‘ingredients’ used to build coral skeletons. Therefore, you need to keep alkalinity in the appropriate range in a reef tank to ensure 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 trying to stay consistent once you find 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. So do what you can to maintain stable alkalinity. Even if you are trying to get your alkalinity up, you will want to do so very, very gradually to avoid shocking any of the animals in your system.

2: Ammonia

That same chemical in your glass cleaner that helps make your glass shine and stay streak-free? It’s inside your saltwater aquarium and comes in as the number 2 reef tank parameter to watch.

Ammonia occurs naturally as a byproduct of the organic waste breaking down in your tank. Ammonia gets into your reef tank when your fish pee (yes, normal biological functions apply to fish, too) and when food and other biological components (plants, live rock, etc.) rot. So it’s perfectly natural to see it in your tank. The problem is ammonia is toxic waste in your aquarium. In chemical terms, ammonia is a base or a basic compound – the opposite of an acid.

But while acids and bases are opposites, they cause a similar burn when they contact living things. The ammonia in your tank will burn your saltwater fish and corals and possibly 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. However, if you have detectable ammonia levels in your tank, it means your aquarium is too new (it has not fully cycled yet). Or you could have a problem with your biological filter.

Ideal Ammonia Parameter for a Reef Tank

As another 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. This is why there is no tolerance for this in a reef tank. The ideal ammonia level is ZERO.

3: Calcium

Have you seen a milk commercial on television promoting calcium for good bone growth? Well, fish aren’t mammals and don’t drink milk. (Now that I think about it, bringing up milk didn’t really help this story much) Moving on!

Calcium is an essential element in a reef tank for Small Polyp Stony (SPS) or Large Polyp Stony Coral (LPS) health. While they don’t have bones, they have bony skeletons made from calcium, and they get that calcium from seawater.

Calcium is the 20th element on the periodic table

Calcium might be 20th on the periodic table, but it’s third when it comes to reef tank parameters

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

Ideal Calcium Parameter for a Reef Tank

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

4: 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. It means your biological filter is working. (News flash: you want that!)

nitrate test

Nitrate test at work

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

The problem crops up when your aquarium gets more crowded and mature. More and more nitrate gets generated, allowing the levels to climb. Small amounts of nitrogen in the nitrates get absorbed by certain soft corals or macroalgae. But it also acts as a fertilizer, boosting problem algae growth. (Nobody wants that!)

Some invertebrates are intolerant of high nitrate levels. Their health will decline (adding even MORE nitrates) if left unattended.

Ideal Nitrate Parameter for a Reef Tank

It is best to keep nitrate levels as close to 0ppm in a saltwater aquarium. However, you may 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 tend to come from nutrient-rich waters. 

Reducing Nitrates in 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.

However, to help keep your nitrates low on an ongoing basis, some aquarists employ the help of beneficial bacteria which eat the nitrates. They accomplish this via a process called carbon dosing. What is carbon dosing, you ask?

The short version is to allow beneficial bacteria to naturally grow in your tank. But you can give those 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.” (Sorry to disappoint)

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. Though they ARE related to one another through the nitrogen cycle.

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 into less toxic nitrite before the second group of bacteria converts the nitrite to nitrate. (See? It all makes sense in the end)

Ideal Nitrite Parameter for a Reef Tank

For a very brief period of time (a few days, MAX) – say, while you are cycling your tank – you look for the presence of nitrites in your tank. This provides 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’s a symptom that A) your biological filter has crashed or B) it can’t keep up with SOME large source of ammonia (say, a dead fish you haven’t located yet).

6: pH Level

I won’t bore you too much with the scientific definition of pH. It’s a long, drawn-out explanation that revolves around hydrogen ions. The pH scale is logarithmic, meaning that 8.0 is MUCH farther from 7.0 than you think. 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 the chemistry of your reef tank, which is why pH ranks as one of the most important reef tank parameters.

Ideal pH Parameter for a Reef Tank

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 it 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.

 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 trouble-maker, even at natural levels.

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.

Ideal Phosphate Parameter for a Reef Tank

Determining the ideal level of phosphates for a reef tank is tricky. 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 a nutrient.

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

To keep your levels low, you have a couple of easy solutions:

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, I’m referring to the amount of salt in the saltwater. (I know, crazy concept) But salinity is difficult to measure directly.

One way to measure the salinity of the water is to completely evaporate a liter of saltwater and weigh the salt left behind. That would be your salinity. However, it’s not the most practical way to handle things with a reef tank.

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

When the salinity of the water changes, two other things fluctuate at the same time:

So, aquarists use a hydrometer to measure the water’s density (specific gravity). We also use a refractometer to measure the angle of light refraction to estimate the salinity.

The most common type of hydrometer used in this hobby is called the swing-arm model. It looks like this:

saltwater aquarium water parameters testing

Hydrometer for measuring density to estimate salinity

You measure the salinity by submerging the device and filling it with your aquarium’s water. The arm is carefully calibrated to float and point at the water’s corresponding specific gravity and salinity.

The other way of measuring salinity is with a refractometer.

Refractometers are more precise and less likely to fail. However, they require you to occasionally calibrate them to maintain the desired precision. This is what a refractometer looks like:

refractometer is sometimes used to measure salinity of reef tank water

The amount of salt in reef tank water predictably refracts light allowing us to measure salinity

You have to have salt in the water to create salinity. (Sounds reasonable, right?) So most of us make our own seawater with a salt mix.

Ideal Salinity Parameter for a Reef Tank

The salinity of the ocean comes in at ~ 35 g/L (using the evaporation method). But hobby aquariums measure the specific gravity of the water because of the ease of the measurement. Ideally, you want to keep your aquarium reef at a specific gravity of  1.025. This is the simplest way to ensure the salinity replicates the salt concentration of a natural reef.

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 between 73 and 84 degrees Fahrenheit (22 to 29 degrees Celcius).

If you live in an area where the ambient temperature matches that range…Well, I’m a bit jealous and hope you’ll invite me over to hang out very soon. If you don’t live in an area with a steady tropical temperature, you will need equipment to keep the aquarium temperature in the right zone.

Thermometer noting typical range

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


An aquarium heater is required for just about any aquarium. Unfortunately, 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 optimum levels.

If you live in a warm climate, where the temperature gets above the ideal range, you may need an aquarium chiller.

Ideal Temperature Parameter for a Reef Tank

Since most of what we do is intended to recreate the natural environment our animal friends come from, it’s no surprise the ideal recommended temperature for your reef tank is the same. The recommended range is 73-84 Fahrenheit (22-29 Celcius).

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

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

The 9 Most Important Reef Tank Parameters

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