Thursday, May 08, 2014

How to Measure Salinity in a Saltwater Aquarium


It goes without saying that salinity is the most fundamental parameter in a saltwater aquarium. Without salinity, it isn’t saltwater.

Yet, perhaps precisely because salinity is such an important parameter, it is often overlooked. This article will explain what salinity is and why it is important to measure salinity. Then, we will discuss what your salinity level should be, and how to test for it.

Salinity is the measure of dissolved salt content in water. Salinity is typically measured in parts per thousand (ppt) or specific gravity (sg). The more salt in your water, the higher your readings will be. Saltwater aquarium salinity is usually kept between 1.020-1.028sg. Some aquarists like to keep fish only aquariums at lower levels (1.017+) to keep salt costs low and to reduce problems with parasites, which don’t survive as well in lower salinity.

Many reef aquarists like to maintain salinity at higher levels, sometimes as high as 1.031 because salinity tends to be higher around a reef. Choose what is right for your aquarium, and make sure it stays stable. An auto top off system can alleviate your troubles if evaporation is causing your tank to fluctuate a lot day to day.



Salinity should be checked very regularly. Oceans are very stable environments, where there is not a lot of day to day change in the water parameters. Fish can respond negatively to even small changes in salinity. So, if you have let your salinity get out of whack, make gradual changes. Then, redouble your efforts to keep everything steady.



The most economical way to measure salinity is a hydrometer. There are two basic types: glass models and plastic boxes.

A glass hydrometer floats in the water. The glass tube will sink to a certain level, depending on the density of the water. You then read the measuring line on the tube. If you use one of these models, please do your testing outside the tank. Water flow makes it impossible to measure accurately in the tank. Beyond that, glass breaks. You do not want a broken hydrometer spilling glass and innards into your precious tank. Fill a drinking glass with tank water and test that way. Twenty years ago, glass hydrometers were the standard, but are rarely used anymore.



A box hydrometer is a plastic device with a swing arm in it. When the box is filled with saltwater, the swing arm rises to a certain point, depending on the salinity. Box hydrometers are inexpensive and easy to use, but often inaccurate. If the swing arm is corroded or has dried salt on it, the measurement may be off. Most boxy hydrometers cannot be calibrated. When you are looking for stability and accuracy, this can be a problem.

As you can see, the low cost of these models comes with a price. You will not find many dedicated hobbyists who rely on these devices. With what we pay for livestock, a good refractometer can pay for itself with one prevented death.



A good refractometer is a precision instrument for measuring salinity. Without getting technical, a refractometer works by measuring how much light “bends” as it passes through the water. As the salinity in the water changes, so does the angle of the refraction, or “bend.” Be sure you are buying a refractometer made for measuring salinity in a saltwater aquarium. Refractometers can be used to measure any number of things, and are calibrated depending on what they were manufactured for.

When you buy a refractometer, and periodically afterwards, you should check the calibration of your instrument. You can use salinity calibration fluid,  or you can use distilled or RO/DI water, which should measure 1.000sg. When purchasing a refractometer, you may want to consider a unit with automatic temperature compensation (ATC). Specific gravity changes depending on temperature. A unit with ATC specifically made for aquarium use will be the most accurate choice.



To use a refractometer, place a few drops on the glass plane and then place the clear cover over the sample. Point the refractometer at a bright light source, like your tank. Look through the eyepiece in the back to get the measurement. Your salinity is wherever the colored half of the viewfinder stops.




Now, if looking through an eyepiece feels a little too 20th century for you, then you might want to consider a digital refractometer. Digital refractometers work on the same principle as mechanical models, but are more precise. A digital refractometer can also provide readings in the newer standard Practical Salinity Units (PSU) as well as PPT and S.G. You simply drop a sample in the test well on the unit, and it reads you back the salinity. These models are highly accurate and are not significantly more expensive than mechanical models.


There are other digital devices to measure salinity as well. For example, a conductivity meter will measure how conductive saltwater is. You can purchase hand held units like the Tunze Conductivity Monitor. Or, if you have an aquarium controller, you may be able to add a salinity probe. This is a great choice for the dedicated hobbyist. Not only are you able to spot check the salinity level, but you can measure the change over time so that you can ensure everything is stable.


So, there you have it. Salinity is a key parameter to measure for in a saltwater aquarium. Salinity should be one of the first and most frequent tests you do on your saltwater tanks. As always, if you have any questions one of our team members is always happy to assist.