How to Maintain Salinity in a Saltwater Aquarium

How do you maintain salinity in a saltwater aquarium?  This is one of the most important questions when it comes to reef aquariums.  Keep on reading to find out how and what you need in order to correctly measure the salinity levels in your saltwater tank. 

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What is the Proper Salinity for a Saltwater Aquarium?

Salt concentration is measured in Specific Gravity or Salinity

– Ideal specific gravity for a reef tank is 1.023 – 1.026
– Ideal salinity for a reef tank is 33 – 35

(If you’d like to know which salt I use in my saltwater aquarium and why, then read this article.)

How To Measure Salinity in a Reef Tank?

A refractometer is used to measure salinity in a saltwater aquarium. 

What is a Refractometer?

refractometer looks a lot like a monocular but has been adapted to include an angled glass surface on the rear end with a hinged cap. It works by placing your aquarium water onto the glass surface with a provided pipette and lowering the cap onto the liquid. Holding the lens to your eye you look through it towards a light source, and establish what your salinity is on an imprinted scale within the device to ensure that your saltwater aquarium salinity is not too high or too low.

 There are many brands of refractometers of the market, and not all of them are used for the purpose of testing the salinity of a marine tank.  

Refractometers are used for wine making, beer making, and testing sugar content.

It is very important that you spend your money on a refractometer that has been specifically produced for the saltwater aquarium trade to ensure you provide the correct salinity for your inhabitants, such as this one:

Salinity Refractometer for Seawater and Marine Fishkeeping Aquarium 0-100 PPT with Automatic Temperature Compensation
  • Measure salt water or salinity of water, ideal for aquariums and marine monitoring.
  • Measures on 2 scales, Specific Gravity (D 20/20) and parts per thousand.
  • Features automatic temperature compensation.

Refractometers are fairly easy to calibrate, but the ATC (Automatic Temperature Compensation) tends to confuse people.  Essentially, it has a central core comprising of a bimetal strip which adjusts the unit to compensate for temperature fluctuations.

In laymans terms, once you’ve calibrated it correctly, it does not matter what the ambient room temperature is (as long as they are not extreme), it will still give you an accurate reading.

How to Calibrate a Refractometer?

The refractometer will need to be calibrated at a room temperature of 68 degrees farenheit or 20 degrees centigrade.  This will ensure that the ATC works correctly at any temperature.

Firstly, you place RODI water on the glass surface and look through the view finder.

The imprinted scale you see within the device will be horizontally divided into a blue field and a white field. By using a screwdriver which is included you adjust a small dial until the the horizontal division falls directly in line with the ZERO line.

The blue field will fill the lower part of the scale beneath the ZERO line. The refractometer is now calibrated.

Calibration can also be done using calibration fluid instead of RODI water.  

How to Read a Refractometer?

When you measure your aquarium water the blue field and white field divide should move further up the printed scale within the device to coincide with your salinity.

The frequency at which you check your salinity will depend on evaporation, water changes and system volume in your saltwater aquarium.  A larger aquarium’s system is far less likely to fluctuate than a smaller system.

What if the Salinity is Too High in my Saltwater Tank?

The salinity of your aquarium increases in two ways

Incorrect Dilution Rates

If you have mistakenly mixed a solution of salt water at a higher salinity and added it to your tank without realising, this will raise your salinity.



This is the number one cause of your salt water aquarium salinity being too high. When evaporation occurs, it does not take salt with it. Consequently you are left with a higher reef tank salinity because you have a smaller volume of water in your tank with the same amount of salt content.


What sea salt do I use? 
Check out the latest price 
here on Amazon.

How to Lower Salinity in a Saltwater Aquarium

The salinity of seawater is between 34 and 36. The way in which you reduce the salinity in your marine aquarium hinges on your situation. It is never a good idea to reduce the salinity suddenly. The recommendation is that you put a plan in place to reduce the salt content over a few days.

From experience, I have had to reduce my salinity from a salinity of 38 back down to 35. My aim was to reduce it from 38 to 37 on day one, 37 to 36 on day two, and finally 36 to 35 on the third day.

This is simply done by removing aquarium water, and replacing it with the same amount of RODI water.  Then test your KH/Alkalinity, Calcium (Ca) and Magnesium (Mg) to ensure they have not drastically reduced.

Refractometer vs Hydrometer?

One of the first lessons marine keepers will learn is that skimping on the essentials is not an option.  One such decision is how to measure your salinity or specific gravity.

A hydrometer is submerged directly into your tank.  It utilises a needle within a compartment which rises, giving you a reading along it’s numbered scale.  There are however some drawbacks to using a Hydrometer.

To establish as accurate a reading as possible it has to be kept dead-still in your tank to ensure the needle does not bob.  Even when holding or securing it to ensure there is no movement, the flow of the water or the flick of a fish fin and that needle will bob.  This is a cause for concern because it will never settle on a definite reading.  What you are estimating to read 1.026ppm may well be 1.030ppm!!

The hydrometer also needs to be thoroughly cleaned to prevent salt from forming within the compartment, which in turn causes the needle to become stuck.  Bubbles can also be an issue so you will need to ensure that the device is bubble free.

Finally, you just hear far too many complaints of hydrometers being just plain inaccurate, full stop.

Hydrometers are normally included in beginner setups, giving newcomers the impression that they are the tool of choice. My advice would be to take that ‘tool of choice’ and put it in the bin.

So there you have it!

Salinity stability can be tricky to achieve in the beginning, but once you’ve figured out the evaporation rate of your tank you can counter its affects proactively.

Maintaining a stable salinity becomes far easier when you invest in an ATO (automatic top-off) unit.  An ATO automatically detects your water level and tops it up through the day using RODI water.  You can read more info on auto top off units here.  

Saltwater Aquarium Salinity