Aquarium Chemistry: Salinity

In the previous edition of Aquarium Chemistry:  TDS, the proper water quality required was discussed.  Next, it is critical to discuss the concept of salinity.

Specific gravity is often the standard measurement and is the ratio of the density of a fluid relative to water at a given temperature (a reef tank usually has a specific gravity around 1.026).  It may also be measured in parts per thousand (ppt), and a typical reef tank has a salinity of 35 ppt.  Additionally, the conductivity of the fluid may also be measured.  Measurements may be taken with a hydrometer, refractometer, or an electrical conductivity meter.


Hydrometers work on the principle of water displacement.  As the density of a fluid increases, a known mass will displace the fluid relative to a scale.  Although hydrometers are fairly precise (likely to give a similar answer on repeated tests), they are not very accurate (likely to give the correct value).  This is due to the factory calibration.  Years ago, most hydrometers were calibrated at 60 degrees Fahrenheit, which is rarely the temperature at which hobbyists test their aquarium water.  Since the density of fluids change with temperature, a temperature other than 60 degrees will affect the result.  Thankfully most manufacturers have caught on to this issue and calibrate their hydrometers at a typical reef temperature.


Refractometers measure the salinity based on a ratio of how fast light travels through a vacuum compared to how fast the light travels through the fluid.  Since light will travel slower through the fluid, a number greater than 1.000 will be given.  Refractometers are very precise and much more accurate than hydrometers if used properly, but they also cost more.  Calibration of a refractometer involves using a known salinity test, such as pure water and a 35 ppt solution.  Again, it is important to use fluids to calibrate at the same temperature as the tank’s temperature.

Electrical Conductivity Meters

Electrical conductivity meters measure the ability of the fluid to conduct electricity.  A probe measures the difference in voltage between two electrodes.  Again, temperature variations do make a difference in the measured conductivity.  This measurement device is generally the most expensive.


  1. Jakc

    How fast can I raise my salinity? I just learned it is too low

    1. admin

      Hi! You should raise your salinity no more than 2ppt a day as this can injure fish. Water changes with the correct salinity water are ideal rather than adding salt directly to the tank. I find this handy calculator is great for adjusting salinity: http://www.saltyzoo.com/SaltyCalcs/SalinityAdjust.php

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