Measuring salinity correctly is an important skill for maintaining a saltwater aquarium because the fish and other animals inside the tank depend upon you as an aquarist to maintain the proper salinity levels, without fluctuations, on a regular basis.

While it is one of the most basic and fundamental parameters in an aquarium, it is extremely important to the health of your tank inhabitants. Hobbyists have a habit of neglecting to monitor salinity because it is such a fundamental parameter that is fairly easy to understand and control. The constant or simple nature of salinity just makes it easy to forget or overlook, especially when diagnosing a problem with your tank. This can quickly lead to some serious consequences for your fish, corals, and other tank inhabitants. 

Seawater Chemical Composition

What Is Salinity?

Salinity is the measure of dissolved salts in water. Salinity is typically measured in parts per thousand (ppt) or specific gravity (sg) but can also be measured using conductivity. The more salt in the water, the higher the salinity readings will be. In an aquarium, we typically are using artificial seawater that resembles natural seawater in terms of chemical composition.  

Seawater is a complex chemical solution that contains almost every known element at varying concentrations. Seawater contains organic and inorganic chemicals and a wide range of trace elements. Some elements exist in more abundance making them more predominant over their influence on salinity. 

Predominant Elements In Seawater

  • Chloride generally exists in about 19,000 ppm
  • Sodium commonly exists at 10,500 ppm
  • Sulfate (approximately 2,700 ppm)
  • Magnesium (roughly 1300 ppm)
  • Calcium (about 420 ppm) 

Changes in concentrations of ions outside of sodium and chloride generally do not affect the salinity measurement as these two elements make up such a large portion of saltwater. Seawater tends to have a natural salinity of 35ppt (parts per thousand), meaning that every kilogram of water has 35 grams of dissolved salts. This corresponds to a specific gravity of 1.0264 and a conductivity of 53 mS/cm.

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 in your aquarium, make gradual changes to correct it and double your efforts to maintain that stability moving forward.

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The Importance Of Salinity

Salinity measurements are used when making or mixing your own artificial seawater and to monitor your tank directly.  

Salt Mix

When mixing saltwater, commercial salt mix is dissolved into purified RO/DI water. The amount of salt required will be adjusted using a salinity measurement, adding enough salt to reach your target salinity level without overshooting it is the idea. Manufacturers will provide particular measurements for how much salt mix is required to reach a particular salinity level which is a good starting point. A salinity measurement is then used to verify that level and adjust accordingly based on your needs.   

Evaporation & ATO

In an aquarium, freshwater evaporates on a daily basis and the salt ions stay behind. This means as H2O leaves the aquarium with evaporation, the salinity level will slowly rise. Aquarists will combat this evaporation by replenishing the tank with purified RO/DI water on a daily basis. This replenishment can be done manually by simply pouring a container of freshwater into the aquarium on a daily basis to maintain a particular water level but that becomes tedious. Instead, most hobbyists employ an Automatic Top Off System instead.

An ATO automatic replenishes your aquarium with freshwater as needed based on the water level.  As water evaporates, the water level drops in your tank which will then trigger the ATO pump to move freshwater into the tank.  The pump is cut off when the water level is returned to normal.  This will maintain a stable salinity level in your tank.  

It is best to check your salinity level in an aquarium at least 1-3 times per week and also before/after your water changes. When using an ATO, things can happen that cause the salinity to drift.  Water gets spilled, the pump fails or stays on too long, and even water movement can cause the ATO to fail and result in a salinity fluctuation. Point is, even though you have a reliable ATO on your tank, DON'T NEGLECT SALINITY MEASUREMENTS.  

Salinity Level In An Aquarium

As a general guideline for saltwater reef tanks, it is best to maintain a salinity of 1.026 (or 35ppt or 53 mS/cm conductivity) and but also know where your aquarium fish come from to decide what their natural salinity level is.

  • Brackish Water Tanks: 1.007 - 1.013
  • Hyposalinity For Parasite Control: 1.009 - 1.011
  • Fish-Only Saltwater Tanks (FOWLR): 1.017 - 1.023
  • Reef Tanks: 1.023 - 1.026

As outlined above, it is not uncommon for aquarists to keep fish-only saltwater tanks at slightly lower salinity levels due to the belief that marine organisms are less stressed at lower salinity levels, this may or may not be true. Some organisms may be from geographic origins, such as the Red Sea, which has higher salinity levels, or brackish estuaries, which have lower salinity levels. When deciding your salt level, it is best to know where your aquarium species naturally come from and create an environment closest to their original habitat.

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How to Measure Salinity

There are few different devices hobbyists are using to measure salinity. Hydrometers are classically the most economical options but leave some room for error. Analog Refractometers are by far the most popular for they are affordable, easy to use, and fairly reliable. Digital meters are also available and will either use light refraction or conductivity to measure salinity and have gained popularity in recent years for their ease of use and reliability.

Swing-Arm Hydrometers

There are two types of hydrometers, box or swing-arm hydrometers and floating glass hydrometers.

A box hydrometer is a plastic box with a salinity scale printed onto it and a swing arm inside of it. When the box is filled with saltwater, the swing arm rises to a certain point, based 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 swing-arm hydrometers cannot be calibrated and must measure saltwater at a predetermined temperature to maintain accuracy.  

Floating Glass Hydrometer

Floating Glass Hydrometers

Better than a swing arm, a glass hydrometer is a special glass tube with a salinity scale inside, much like a thermometer.  The glass tube floats at a certain depth based on the salinity of the water.  Water density increases with salinty causing the glass hydrometer to float higher in the water with increasing salinity. You then read the salinity using Specific Gravity (SG) at the water line where it lines up on the scale.

These are great to have on hand for mixing saltwater and can maintain accuracy so long as you care for them. Being glass, fragility is an issue but they also must be used in a separate container. Water movement makes it impossible to use a floating glass hydrometer correctly so a separate beaker, graduated cylinder or tall container must be used to sample your water and take the measurement.  

Analog or Mechanical Refractometer

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.” Refractometers are classically the most popular choice for saltwater aquarists, they are reliable and can be calibrated while also being affordable and easy to use.  They will provide readings using Specific Gravity with some of them also containing a Parts Per Thousand scale. 

When using a refractometer, a drop of salt water is placed onto the lens. Close the cover and point the refractometer towards a light source, like your tank or window.  Inside, a salinity scale is present and there will be a line of color change that aligns with your water's salinity level. Accuracy is as only as good as your ability to read that scale and color change. 

When using a refractometer, checking calibration often is necessary.  A salinity reference solution should be used to check calibration before every single use and if you find the readings are incorrect, calibration should be performed before use. 

Also, be sure you are buying a refractometer made for measuring the salinity of seawater; refractometers can be used to measure any number of things, and are calibrated depending on what they were manufactured for.

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Digital Refractometer

Digital refractometers, work on the same principle of measuring light refraction but the results are digital instead. This eliminates some human error by removing the need to look through a viewfinder but they still require frequent calibration checks. Digital refractometers have become more affordable over the years and are a great choice for tank owners.  

Digital Salinity Meter

Digital salinity meters are rising in popularity because the technology is becoming more affordable and are probably the easiest solution for a modern aquarist.  They work using conductivity (reliable) and can provide readings in Specific Gravity, Parts Per Thousand, and even Conductivity mS/cm in some cases. The readings are almost instant and you can quickly and easily check calibration before each use using 35 ppt calibration solution.

The salinity probe on your aquarium controller like the Neptune Systems Apex works using conductivity too. There are also specific conductivity meters that will provide the results using mS/cm conductivity which can then be converted to Specific Gravity or PPT with a formula, but who likes math these days?