Reef Tank Hardware

Hydrometer vs Refractometer – Which Is Better For A Saltwater Aquarium ?

Hydrometer vs Refractometer – Which Is Better For A Saltwater Aquarium ?

The single most important parameter in any reef aquarium is salinity.  Without a stable salinity of 35ppt or 1.026 specific gravity it is very difficult to maintain healthy coral growth.  If a tank’s salinity is not correct then every other major and minor element will almost inevitably be out of the optimal range.  This means that salinity should be checked on a regular basis wih a properly calibrated device.  The two most popular methods for checking salinity are hydrometers and refractometers.  While both will effectively measure salinity there are some key differences in how they work as well as cost and accuracy.



A hydrometer, such as this one from Instant Ocean, is a quick and inexpensive method of measuring salinity.  While they are not as accurate as a refractometer, they are less expensive and some find them simpler to use.  A hydrometer operates on the principle that salt water has a higher density than freshwater.  By calibrating the hydrometer to read 1.0 specific gravity when filled with freshwater and then filling the hydrometer with tank water, a salinity reading can be obtained in both ppt and specific gravity.  This can be done in a matter of seconds to allow for regular monitoring of salinity.




While a hydrometer can give a fairly accurate salinity reading, the gold standard for measuring salinity in terms of both accuracy and durability is a refractometer.   It is true that a refractometer has a higher upfront cost, however, a well made device such as the Reef Casa Refractometer will likely outlast most tanks if properly cared for.  A refractomter uses light to determine the density of saltwater.   This is because the angle of refraction, or light shift, varies depending on the density (and therefore salinity) of the water.  Lenses and prisms are used to project a line on a salinity scale which is then viewed by the hobbyist through an eyepiece.  While this may sound complicated it is actually quite simple and all it really takes is one YouTube video or in person demonstration to get the hang of it.  In addition to being longer lasting, a refractometer will also provide a more accurate reading than a hydrometer.


While we recommend refractometers either device will work to measure salinity in a reef tank.  Regardless of which one you choose, the key is to keep it calibrated and check salinity regularly.