We all know seawater contains salt, along with a variety of other elements. In order to keep your fish healthy, you need to maintain a precise amount of salt in the water which we measure using salinity or specific gravity.  It gets a little complicated because these two common methods are completely different but ultimately can give you the right information.  

Salinity vs Specific Gravity

Without getting overly scientific, here are the definitions of these two methods of measuring the salt content of the seawater in your aquarium.

  • Salinity: The amount of dry salt compared to the amount of water (H2O). Measured by weight and expressed in PPT.
  • Specific Gravity: Otherwise known as "relative density" is the ratio of the density of seawater compared to the density of pure RO/DI water. 

That alone doesn't give us much to work with so let's look at these measurements in the application. Salinity is measured in PPT or parts per thousand. Seawater measures 35 PPT which means 3.5% of the weight of the seawater is due to the salt content.  

Specific Gravity does not have a unit of measure and is a ratio simply expressed as a numeric value. The more salt in the water, the denser it gets. Seawater specific gravity is 1.026, and so long as the seawater you're measuring is mixed with the proper ratio of the right elements, we can infer the salinity measurement of seawater by calculating density or specific gravity.  We use hydrometers to measure specific gravity which are calibrated to measure the density of seawater at a particular temperature.

Since density changes with temperature, the water temperature is important when using a hydrometer. As temperatures increase, the density decreases so the same sample of water can measure differently depending on the water temperature. Most aquarium hydrometers will be calibrated to read accurately with water temperatures of 68° to 85° F. 

Tools for Measuring Salinity

Hydrometer: Measures density and is expressed as specific gravity. As long as the sample of the water has the correct ratio of salt and elements, we can infer the level of salinity from these measurements. Seawater measuring 1.026 Specific Gravity has a salinity level of 35 PPT. Seawater hydrometers usually have a scale for both Specific Gravity and PPT.

Refractometer: Measures light refraction to determine the salinity. The more salt in the water, the more refraction you get. Seawater refractometers are calibrated to measure seawater specifically and usually have a scale in both PPT and Specific Gravity. There are both analog and digital refractometers available to hobbyists. 

Salinity Probes/Meters: These devices are actually measuring conductivity which is the water's ability to conduct electricity. The more salt in the water, the more conductive the water is. Using the conductivity measurement we can calculate an accurate salinity measurement using a simple formula. Most "salinity meters" simply calculate the salinity for you and express the measurement in PPT.  

Which Salinity Tool Is Best?

All three methods work but have certain limitations. No matter what approach you choose, you want to use a salinity reference solution to verify your measurements. Hydrometers are inexpensive but cannot be calibrated and you must pay careful attention to the water temperature. Analog refractometers are the mid-range price point and the most popular choice because they can be calibrated which means you can verify they are reading accurately. Salinity Meters and digital refractometers are typically the most expensive but also the easiest; they can be calibrated and give you a reading in a matter of seconds without much effort. 

Using a refractometer or salinity meter is the best choice for saltwater hobbyists. It's not a bad idea to have both where you can use the salinity meter on a regular basis and verify measurements with a refractometer should you question the results.

How To Use an Analog Refractometer

  1. Calibrate the refractometer using BRS Refractometer Calibration Solution
    1. Lift the plastic cover
    2. Cover the prism with 2-3 drops Calibration Solution
    3. Drop lens and ensure liquid completely covers the prism.
    4. Wait 45-60 seconds for the temperatures to stabilize
    5. Look through the viewport while holding the end of the refractometer up to a suitable light source
    6. Focus the reading using the viewport and turn the adjustment screw to set the measurement at 35 ppt.  (it may read accurately right out of the box).
  2. Rinse the tip of your refractometer with RO/DI water and dry it off with a soft cloth.
  3. Cover the prism with a sample of seawater from your aquarium
  4. Drop lens and ensure saltwater completely covers the prism.
  5. Wait 45-60 seconds for the temperatures to stabilize
  6. Look through the viewport while holding the end of the refractometer up to a suitable light source
  7. Focus the reading and take your measurement. 

How To Use The Hanna Digital Salinity Tester

  1. Remove the safety plastic from inside the battery compartment
  2. Rinse the tip in RO/DI water and gently dry it
  3. Cut open the Hanna Salinity Calibration Solution Packet
  4. Turn on the salinity tester and press the CAL button
  5. When the meter displays "35.00 USE" proceed to insert the meter into the calibration solution packet. 
  6. Tap gently to remove air bubbles
  7. Wait until the "Rec" on the display goes away
  8. The display will quickly flash "Stor" and the tester will return to the normal operation mode
  9. Remove from calibration solution and rinse in RO/DI water before taking a measurement of your tank water. 
  10. Insert the probe tip into a sample of your tank water until the measurement stabilizes.

Remember, your tank evaporates freshwater only and the salt stays behind. That means you must replenish the tank with fresh RO/DI water on a daily basis to maintain the salinity level. DO NOT top off your tank using saltwater or else the salinity levels will rise to dangerous levels.