Friday, August 21, 2015

Salinity: How to Measure and Why It's Important




Keeping an accurate salinity level in your saltwater aquarium is crucial to the health of your aquatic friends. Today we are going to explain why it is so crucial to keep a stable salinity level and show you the easiest and most accurate ways to monitor the salinity in your tank.

The "saltiness" or salinity is most commonly measured by specific gravity, which is a somewhat complicated unit of measure that has to do with measuring the density of the saltwater. You will also sometimes see it measured as PPT or part per thousand but, for all intents and purposes, we are going to reference Specific Gravity measurements throughout this video as it is the widely accepted unit of measure in the aquarium industry.

Corals require a stable salinity in the range of 1.023-1.026 specific graving. Most of the fish found in a reef tank are a little more tolerant compared to corals and can handle levels being maintained between 1.020-1.026.


Saltwater fish have a lower salt level inside their body than their surroundings. Because fish are constantly drinking saltwater, they have to actively regulate their internal salt levels which is called osmoregulation. Fluctuations in salinity make the fish have to work much harder to regulate the salinity of the water inside their bodies which in turn causes stress. When keeping corals and invertebrates, it is all that more crucial to maintain a stable salinity level because they are even more sensitive and fluctuations can easily cause them to perish.



In order to maintain salinity levels, many hobbyists will manually top off the aquarium with freshwater on a daily basis which does a sufficient job. After a few months of keeping a reef tank, most hobbyists invest in an automatic top off system (ATO) that adds freshwater to the aquarium multiple times each day to help maintain a stable salinity.

Measuring your salinity level should be done weekly, or at least every time you perform a water change—both before and after changing out the water.


The tool that most beginner hobbyists are familiar with is the swinging arm hydrometer. They are extremely inexpensive and extremely easy to use. However, they can also be extremely inaccurate. Salt crystals, calcium build-up and even air bubbles can cause a hydrometer to provide inaccurate readings. If you are using a hydrometer, we strongly recommend that you check the accuracy periodically with a more accurate device like a refractometer, which we will discuss later in this article.



Another quick and easy tool for measuring salinity is the floating glass hydrometers. The advantage is that they are more accurate than swinging-arm hydrometers and readings will not typically drift or change as the unit is used more and more. The disadvantages to floating glass hydrometers are that they are quite fragile and measurements can be difficult to read because of the small scale.


The best option for measuring salinity is a refractometer, such as the Marine Depot Refractometers or Red Sea Seawater Refractometer available on our website. It is a very simple tool to use and is extremely accurate. A refractometer utilizes a prism and light refraction to measure salinity; different salinity levels causes the light to refract differently.



While they are a bit more expensive compared to hydrometers, they are well worth the investment. A refractometer is very easy to calibrate, can last a lifetime if stored and used properly, and will provide you with quick and accurate salinity level readings each and every time. In fact, Marine Depot staff members prefer the use of refractometers over any other salinity measuring device. I am sure many of you hobbyists out there will agree.

Lastly, we have digital salinity meters and monitors which are great for those of you who love high-tech equipment.



Milwaukee and Hanna both offer a digital refractometer which will give you a digital salinity read out by simply placing a couple of drops of water on the meter. Just like an analog refractometer, these digital units are very easy to use and calibrate.



Conductivity monitors are also a handy way to electronically measure salinity. These monitors measure the electrical conductivity of the water which changes with fluctuating salt levels. The Neptune Systems Apex and many other leading aquarium controllers can allow you to continually monitor and record your aquarium's salinity level via conductivity.

With an option for just about every level of hobbyist, there really is no excuse to have fluctuating or improper salinity levels in your aquarium. Eliminating one possible stress factor to help keep your fish and corals healthy will surely help you on the road to building a flourishing reef.

Thank you for giving us the opportunity to share our knowledge with you. If you found this information helpful, please like and share it to help us spread the word! Don't forget to subscribe to our YouTube channel to stay up-to-date on all our latest video tutorials.

Until next time, take care and happy reefkeeping.



4Read more about measuring salinity in a saltwater aquarium.

4Shop and compare aquarium hydrometers and refractometers.

4Shop and compare aquarium controllers and monitors.