How To Easily & Accurately Measure Aquarium Salinity

Salinity is one of the most important parameters to monitor in your aquarium because without salt, you have a Freshwater Aquarium!
Salinity is the measure of how much salt is dissolved within a body of water, and keeping it as constant as you can is one of your main jobs with a saltwater aquarium.

I remember making the jump from fresh to saltwater and was nervous about all the salt mixing stuff, but once you have done it once or twice it is no big deal. If the salt content is reading low, add more salt. If the salt is reading high, add more water.

Refractometers are the most common tool used by aquarists. Place a few drops of aquarium water on the viewing prism, close the lid and look through the eyepiece. Where the blue and white areas meet is the salinity reading of the aquarium water – 1.024 SG or 30% is the ideal salinity.

Find out why most people start with a hydrometer but soon realize they are not accurate enough for today’s aquarists and why getting accurate salinity readings are so important.

Read On…

How Do You Measure Aquarium Salt Concentration?

There are 3 types of device that are available to measure the Salinity or Specific Gravity (SG) of your aquarium saltwater:

  1. Hydrometers
  2. Refractometers
  3. Electronic Sensors


Hydrometers come in two types:

Glass Hydrometer & Thermometer
Glass Hydrometer & Thermometer
Plastic Hydrometer
Plastic Hydrometer

Glass – These types of hydrometers are floated in the tube which is filled with aquarium water. The hydrometer bobs until it finds its level then the reading is taken from where the water surface meets the numbered line.

These hydrometers often incorporate a thermometer which makes them a handy tool. However, these are not used too often anymore.

Plastic – These are more popular due to their cheap price however they are notoriously inaccurate. Aquarium water is poured into the device then the needle will move until it settles, the reading is then taken from the number the needle tip points to. Air bubbles, dried salt, and crud on the needle can lead to an inaccurate reading. These devices are unable to be calibrated.


These also come in two types:

Optical – These are by far the most popular tool in the hobby and one that needs to be in your toolbox. They are very accurate, easy and quick to use, Can be calibrated and are reasonably priced. Several drops of aquarium water are placed on the glass screen, the lid closed and then you look through the eyepiece to read off exactly where the color change line is. Simple.

Refractometer Reading
This reading would be 1.030

Digital – These are super easy to read, especially for aquarists who find reading the optical refractometer difficult. They are expensive compared to the optical version, but not a silly price.
A water sample is dropped into the sample port and then the reading is displayed on the screen. These can be also be calibrated.

Electronic Sensors:

Get your wallet out!

Electronic salinity testers are awesome, but you pay for it. If you already have an aquarium controller, like the Neptune Apex, then buying the Salinity Module is a great addition for not that much money. For most aquarists, the high cost makes it a want rather and a need. They can also be calibrated and are super simple to use.

What Reading Should You Keep Your Aquarium Salinity At?

Because salinity is the measure of dissolved salt in the water of your aquarium, you cannot see it, so we need to use a device to measure the water and tell us what the current level is.

Dissolved salt content is usually expressed in PPT ( Parts Per Thousand ) and by SG ( Specific Gravity ).
As aquarium owners, we do not use PPT as to get a true reading, you would have to take a known amount of water from the aquarium, evaporate off the water, leaving just the salt and then measure the weight of said salt. This process is known as ‘Mass Solids Analysis’ and is way above everyone’s heads!

Luckily there are some really simple products that measure the SG of the water and provide us with a simple reading!

Salt Values:

There are two number ranges that apply to us fish keepers:

Fish Only Aquariums:

SG: 1.017 – 1.025

For FO aquariums you can get away with having a lower salt concentration in the water, as fish have a higher tolerance to lower SG levels compared to corals. Many owners like to keep their Specific Gravity lower for 2 reasons:

  1. Less salt used = Lower running costs
  2. Lower SG levels are not tolerated by a lot of fish parasites, thus enabling healthier fish

Reef Aquariums:

SG: 1.023 – 1.028

When you delve into the awesome world of corals, your salinity concentration will need to be higher and kept constant. Corals are very picky when it comes to their water and if it is not maintained well, they are going to die pretty quick! Maintaining a rock-solid salinity level is achieved in two ways:

  1. Replacing evaporated water – Usually with a device called an ATO (Automatic Top-Off System)
  2. Regular water changes with identical water parameters

What Salt Do You Need For Your Aquarium?

Salt is salt, right! Well, yes it is and adding salt to water will make saltwater, but you need the right stuff to allow your aquarium to thrive.

Salt is the common man’s or woman’s term for Sodium Chloride. This means it is made up of Sodium ions and Chlorine ions, however, depending on the type of salt, you can have other ions in there too.

Table Salt Shaker
Table Salt

Table Salt, for instance, is high in iodine ions. Adding this to your saltwater aquarium will raise the SG but it will also raise the iodine level too, eventually poisoning your livestock.

What we need is a blend of ions that are perfectly matched to not only provide the right ion balance within the water but to also provide the beneficial ions and elements that fish, invertebrates, and corals use to grow.

In addition to Sodium and Chlorine there are some 20+ more elements that you will find in many of the popular aquarium salt mixes:

  • Calcium
  • Magnesium
  • Potassium
  • Strontium
  • Sulfate
  • Aluminum
  • and on and on…

Each piece of the salt mix blend is expertly added in the correct ratios so all you have to do is measure out the salt, dump it in the mixing vessel, mix, heat and then add to the aquarium.

There are also different salt blends to suit the aquarist. You can get mixes there are aimed for fish and minimal corals and you can get mixes aimed at heavily stocked coral tanks. The ratio of elements in each of these mixes are tailored to help meet the element consumption rates of fast-growing LPS and SPS Corals.

There are many blends of aquarium salt on the market all with varying mix ingredients and varying prices. You can buy salt in bags for mixing 25 gallons of water – good for Nano-Reefs, and you can buy the buckets and boxes for mixing 150 & 200 gallons respectively. The more you buy, the cheaper it is.

I began with regular Instant Ocean but as my corals began to grow I found the added quantities of Carbonate, Calcium, and Magnesium that Reef Crystals provided was worth the extra $8. Reef Crystals is all I use.

How To Maintain Steady Aquarium Salt Concentration?

Firstly, is to make sure your refractometer or sensor is calibrated regularly. I always keep mine in its box, on a shelf above my sump so it doesn’t get knocked. I also calibrate it every 3-4 weeks. Regular calibration is important!

What are the main reasons why your salt level will fluctuate?

  1. You prepare a new salt mix and the salinity level of that mix is off
  2. Evaporation of water from your tank.


This is the biggest reason for fluctuating salinity levels in your aquarium! My 90 gallon system evaporates around 2 gallons of water every day!

The one thing to remember is that Salt Does Not Evaporate!!! When replacing water lost to evaporation, you only ever ADD FRESHWATER!!!
Many people have crashed their tanks by adding more saltwater and all this does is increase your salinity level to a crazy high value that is too much for the livestock.

Myself and probably every other Reefer will have installed an ATO ( Automatic Top Off ) system to their aquarium and let that keep the aquarium topped up with fresh water every day. They are a lifesaver!

==> Click Here To Read My Article On Automatic Top Off Systems <==

How Do You Calibrate An Aquarium Salinity Testing Device?

Refractometer Calibration

Ensuring your testing device is reading accurately is essential otherwise there is no real point in testing! If your testing device is off by a couple of 1/1000’s then you could end up seriously harming your livestock.

Luckily for us, there is a cheap product on the market called Calibration Solution. Its a chemically made to read 35ppt/1.026.

You may notice on the calibration bottle the solution is set to be read at 25°C however when you look at the scale below on your refractometer you may see the values circled as either 20 or 25. These represent the number the refractometer scale was calibrated to, so now what do you do if your solution says 25 and your refractometer says 20?

Refractometer Scale 1

Refractometer Scale 2

When I found this problem I did quite a lot of research to make sure I was getting the test correct and many of the scientific papers I found referred to concentrate on the refractometer being the correct temperature and not worry too much about the solution. So long as the solution was room temperature you were OK for an aquarium.

Why? The small drops of solution you place on the glass of a refractometer will rapidly cool down to the temperature of your refractometer. The difference in readings is minimal so this satisfied me.

To Calibrate You Follow This Sequence:

  1. Ensure your refractometer has been stored at 20°C/roughly room temperature for at least 30 minutes
  2. Ensure the glass screen is clean
  3. Place enough drops of calibration fluid to fill the glass screen
  4. Close the clear lid
  5. Look through the eyepiece and see what reading you get
  6. If you are not getting 35ppt or 1.026 then you need to adjust the refractometer
  7. Using the supplied screwdriver adjust the screw until the reading is 35ppt/1.026
  8. Clean the glass screen and retry with new fluid to confirm reading is correct
  9. Calibration now complete
Refractometer Calibration Adjustment
Refractometer Calibration Adjustment Screw

Electronic Salinity Sensor Calibration

Each manufacturer will have a different process for calibrating their probes. You may require some of the calibration fluid above or you may just need some RO water. Be sure to read the instructions provided by the manufacturer to ensure your sensor is properly calibrated.

How Do You Remedy Incorrect Aquarium Salt Concentration?

If you measure your salinity and you find it is way off, the first thing to do is not panic! A rapid change in any parameter in a saltwater aquarium will always lead to trouble. Your tank has slowly got out of whack, so now you have to slowly bring it back to perfect!

If your reading is higher than 1.025 then you have Too Much salt in your water.

If your reading is lower than 1.025 then you Don’t Have Enough salt in your water.

To remedy this you will need to adjust the salt content on your next water change batch. You need to adjust this level slowly over the next few water changes. A little change at each water change!

If your current aquarium salt level is, for instance, 1.028, then you mix your salt batch to 1.027, then the next batch to 1.026 and so on until your aquarium water finally reaches your desired setpoint. Mine is 1.025 on my reef.

Once you have reached your target setpoint ALWAYS measure your aquarium salt level before mixing your new batch, then you can adjust the salt content in the new batch to suit.

Further Reading

if you found this article helpful you might also like these too:


Hi, I'm Richard and I have been an avid aquarist for over 30 years with a passion for Saltwater Aquariums. I love to pass on my knowledge to help others get the same amount a pleasure out of this hobby as I do. View my About Me page to find out more about me & my mixed reef aquarium.

Recent Posts