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Last update: July 20, 2021

Salinity: What Is the Ideal Saltiness of Your Aquarium?

I don’t need to tell you that the ocean tastes salty.

After all, they call it salt water for a reason.

But did you know that fish need the ideal “saltiness” to survive?

If the salt levels are out of whack, you risk harming your fish.

And, you don’t want that!

You want the saltiness of your aquarium to match that of your fish’s natural environment.

While the perfect salt levels come naturally in the wild, your aquarium needs a human touch.

But how do you determine the ideal saltiness?

That’s where testing for salinity comes in.

And today, I am going to teach you everything there is to know about salinity.

What is salinity?

Fish swimming on reef with crystal clear salt water

While the water in your aquarium may look clear, it actually contains dissolved ions – salts – that are invisible to the eye.

You can replicate this by taking a pinch of table salt and dissolving it in water. Even though the salt disappears, it remains in the water – which is obvious if you taste it.

If you then boil that water until it evaporates, a salt residue will be left behind. This residue is made up of the ions that were dissolved in the water.

It doesn’t matter if it’s seawater or fresh water, all natural water sources on Earth contain these dissolved ions in varying quantities – some ions exist in very high amounts while others are present in very low amounts.[1]

Seawater, for instance, is mostly made up of sodium (Na) and chloride (Cl) ions, among others. Together these two ions give seawater its “salty” taste and account for 85% of all dissolved ions in seawater.[2]

However, even ions that are only present in very low amounts can still be important to aquarium fish and plants.

Salinity is the measure of these dissolved ions.

How is aquarium salinity measured?

Salinity measuring device

You will most commonly see salinity measured in parts per thousand (PPT or ‰).

This is the physical amount of salt in the water.

The generally accepted salinity of natural seawater is 35 PPT.

So, a result of 35 PPT would mean that there are 35 parts salt to every 1,000 parts of water.

To correctly measure the salinity, you need to take a precise weight of seawater, evaporate the water and measure the weight of the residue left behind.

Now as you might have guessed, this is an awful lot of work for the average hobbyist, who just wants to raise happy and healthy fish…

I know that if this is how I had to test the salinity of my tank, I would have thrown in the towel long ago. I don’t have time for that, do you?

That’s why fish keepers use indirect methods to measure salinity.

And here is where it gets a little bit confusing.

You see, the unit of measurement that is used when determining salinity entirely depends on the device used to measure it.

Refractometers and hydrometers measure the specific gravity (SG) or practical salinity scale (PSS or PSU) while salinity meters measure the conductivity in millisiemens per centimeter (mS/cm or mmho/cm).

To a beginner, this can all sound terribly confusing. And, I understand!

For now, all you really need to know is that each of these measurements can be used to determine the salinity of your aquarium.

What salinity is best for your aquarium?

Saltwater marine reef tank

The ideal salinity for your aquarium is entirely determined by the fish and plants you want to add.[3]

The idea is to imitate the salinity found in the natural environment of the fish you keep.

Saltwater aquarium salinity: 35 PPT

Freshwater aquarium salinity: 0.5 PPT or less

Brackish aquarium salinity: 0.5 – 30 PPT, depending on fish

Why do you need to test the salinity of your aquarium?

It’s pretty simple – if the water salinity is too far out of your fish’s comfort zone, it can cause stress or even death.

How often should you check the salinity of your aquarium?

Testing salinity of salt water with salinity monitor and probe

Like most fishkeeping chores, testing the salinity in your aquarium is not a once and done job.

I test the salinity in my tank every time I do a water change.

You know when you mix your salt (I use this) with pure fresh water?

If you add it to your tank without testing it, you risk throwing off the balance of your aquarium.

So before I add my saltwater mix to the tank, I test the salinity level of my mix and adjust it if needed.

The idea is to get the salinity as close to the salinity in your tank as possible, assuming the salinity of your tank is correct.

However, if you are cycling a new tank, I would recommend testing the salinity more frequently – just so you know that everything is going smoothly.

What causes the salinity in your tank to change?

Saltwater nano reef aquarium

1. Evaporation

Over time, the water in your tank will slowly evaporate.

Unless you mark the water level on your aquarium, you might not even notice it happening.

But while water evaporates, salts don’t.

What this means is that the water leaves your tank, but the salts are left behind.

With more salt and less water, the salinity of your tank will be higher.

2. Water changes and top-ups

The most common cause of fluctuating salinity levels is user error.

Either topping off the tank with too much fresh water or failing to test the water you add each water change can seriously mess with salinity levels.

3. Salt creep

You know that crusty salt buildup on the edge of your tank? Well, that is salt that is no longer dissolved in the water.

Over time, this loss of salt can lead to reduced salinity.

4. Other small stuff…

Protein skimmers, new specimens, removing plants and decorations all remove a tiny bit of salt from the water.

While it may not be much, it can add up over time.

How to test the salinity of your aquarium

There are three common tools that aquarists use to test the salinity of their tanks:

1. Hydrometer

Aquarium swing arm hydrometer used to test salinity of seawater

If you want to measure the salinity of your tank, the cheapest way is with a hydrometer.

Seriously, you can pick one up for a couple of dollars.

A hydrometer tests for salinity by comparing the density of the water in your aquarium to the density of pure water – water without dissolved ions.[4]

While they are simple to use, my biggest gripe with hydrometers is that they are the least precise way to measure salinity, even when used correctly.

Since fish can adjust (to a degree) to changes in salinity, a hydrometer may very well be all you need to test salinity – just follow the instruction manual carefully.

But if your tank contains plants, corals and critters like sea stars that are highly sensitive to changes in salinity, then I recommend skipping this option.

2. Refractometer

Analog handheld refractometer with eyepiece to measure aquarium water salinity

Refractometers are by far the most popular way to measure salinity among beginners and experts alike.

Not only are they accurate, but they are also affordable, making them the best value for the money when you want to measure salinity.

Refractometers measure the way light bends through the water to determine salinity.

Analog refractometers require a bit of tweaking to get precise measurements, but not so much that a beginner can’t figure it out.

Fortunately, there are digital options available for those who want to pay a bit more for an easier-to-use device.

Want to know more? Check out our detailed refractometer guide.

3. Salinity monitor

Pinpoint handheld salinity monitor meter for aquarium seawater

Rounding out the salinity measuring devices are salinity meters.

These range from small pen testers and handheld units for occasional salinity tests to automated systems that continuously monitor the salinity and alert you if it is above or below your preferred level.

Salinity meters meters use a probe to test conductivity of the water. To put it simply, the probe records how the water reacts to electricity and uses this reading to determine the salinity of the water.

While salinity meters are easy to use, they are usually more expensive and harder to find than other salinity measuring devices – you likely won’t find one on the shelf of your local fish store.

Aquarium Salinity Conversion Table

Want to convert salinity to specific gravity or conductivity readings? I’ve got you!

Simply use the chart below that covers the ideal range for marine tank salinity.[5]

Reference temperature – 25.0°C / 77.0°F

Salinity (PPT/PSU) Specific Gravity (SG) Conductivity (mS/cm or mmho/cm)
33.5 1.0252 51.0
33.8 1.0254 51.5
34.2 1.0258 52.0
34.6 1.0261 52.5
35 1.0264 53.0
35.3 1.0266 53.5
35.7 1.0269 54.0
36.1 1.0272 54.5
36.4 1.0274 55.0

Conclusion

As you see, salinity is an unusual variable to test for. Not only can it be measured in many different ways but also with many different tools.

However, when it comes to salinity, one thing remains constant – the importance of regular testing, for your fish’s sake.

How do you measure the salinity in your tank? Let me know in the comments below!

Note: PPT is the old measurement of salinity. Salinity is now measured as a dimensionless unit (S). However, I felt the inclusion and explanation of this fact would needlessly complicate an article aimed at beginners, especially given that many salinity measuring devices still include PPT in their scale.

Ian Sterling

I've been keeping fish for over 30 years and currently have 4 different aquariums – it's an addiction. I'm here to teach you everything there is to know about fishkeeping.

I also use this site as an excuse to spend lots of money on testing and reviewing different aquarium products! You can find my reviews here.

Comments (4)

Hi Michael,

That’s good to know. Being able to accurately measure salinity is a vital part of keeping a healthy saltwater tank.

I go with the Apex probe and once a week during water changes, double check with a refractometer to ensure the probe isn’t drifting. When the refractometer and apex probe differ, check with 35 ppt solution on the refractometer and usually it is the one that needs adjustment, haven’t had to calibrate the apex probe, it’s rock solid so far!

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