All You Need to Know About: Salinity

What is Salinity?

Simply put, salinity is the amount of salt measured in water.

Why is it Important?

Saltwater fish and coral have evolved to live in water with a specific concentration of salt or salinity. Most fish and coral can tolerate minor changes in salinity. However, the more significant the salinity change or, the farther away salinity is from the recommended level, the more stress it will cause, eventually leading to death.

What is the Optimal Range?

Different types of saltwater aquariums require different levels of salt. Here are the most common:

Saltmarsh in Virginia
Salt marsh on the Virginia coast

Reef Aquariums: Typical Range: 1.024 – 1.026

Salt concentrations in the ocean’s reefs vary depending on the location. As low as 1.023 and as high as 1.028 are generally considered safe for corals, however, most hobbyists keep their salinity in the range of 1.024 – 1.026 (32 – 35 ppt).

Fish Only Aquariums: Typical Range: 1.019 – 1.023

Generally only used for aquariums with saltwater fish as the salinity is too low for the vast majority of corals. Fish usually tolerate these lower salinity levels; however, parasites and other pests have a harder time coping with this level of salt concentration. For fish with ich, the salinity can be dropped to as low as 1.010 for up to 2 weeks to eradicate the ich infection.

Brackish Aquariums: Typical Range: 1.002 – 1.022

Brackish waters occur where saltwater from the ocean mixes with a fresh water source, like a river. The concentration of salt can vary widely but is rarely the salinity of seawater.

How to Make Saltwater at the Proper Salinity

When deciding to keep a marine or saltwater aquarium, the first thing you may think about is: how do you make saltwater?

  1. Select Salt Mix: There are many different brands of salt on the market. Choose a salt that is a good match for your tank’s needs. For example, if you have a reef tank, choose a reef mix with proper calcium and alkalinity levels.
  2. Make Freshwater: Adding tap water to an aquarium is not recommended as it contains many impurities that can be harmful to tank inhabitants. Purified RO-DI water is considered the best option and can be made at home using an RO-DI filter or purchased from a local aquarium shop.
  3. Mix: After selecting your preferred salt, simply add the salt to the fresh RO-DI water and mix with a pump until the salt dissolves completely. The water should be crystal clear after all the salt dissolves.
  4. Test: Before adding the saltwater to the tank, test the concentration of salt. The most common measurement is specific gravity and should fall in the range of 1.023 to 1.028 for a reef tank.  If measuring PPT or parts per thousand, it should fall in the range of 34-36. (35 PPT is the same as 1.026 specific gravity.) If salinity is low, add additional salt and mix until completely dissolved. Test again.

Tools for Measuring Salinity

There are several tools available in the market for purchase designed to test your salinity.  Here are a few options and the pros and cons of each:

Swing-arm Hydrometer: A swing-arm hydrometer is a small clear plastic box with a tear-shaped swing arm. The calibrated arm will float at a specific height dependent upon the salt concentration of the water.  The scale printed on the box will indicate the specific gravity. A swing-arm hydrometer is affordable; however, it is generally not as accurate as other types. A good option for fish-only aquariums since consistency is more important than the actual salinity level.


Refractometer: A refractometer is more accurate but also more expensive than a swing-arm hydrometer. A refractometer is a hollow tube with an angled piece of glass at the end. Sample water is placed on the glass and then held up to a light source. The sample water density changes the light’s direction, called the angle of refraction, which correlates to a specific salinity value. Results display on a scale inside the tube.  Most economical refractometers measure just the concentration of salt in the water. Higher-end refractometers measure the concentration of seawater, making them more accurate. It is best practice to rinse the slide with fresh water after each use and re-calibrate the device regularly.

Digital Refractometer: Operates with the same principle as a manual refractometer; however, rather than a user’s eye reading the value, the machine will measure the refraction of the light itself and display the salinity as a number on a screen.  As with a manual refractometer, it is best to rinse the slide with fresh water after each use and calibrate it regularly.

How to Maintain Salinity

To avoid changes in salinity, we recommend the following procedures:

  1. Top off for evaporation with fresh RO-DI water only. For tanks with high levels of evaporation, we recommend using an automated water top-off system.
  2. Match the salinity of the current tank water when making new saltwater.
  3. Anytime saltwater is removed, always add the same amount of back to the tank. This rule is essential for water changes and can also be important when adding or removing new fish and corals or when a protein skimmer overflows.

And there you have it!  There are many ways to mix and measure saltwater for your aquarium.  Hopefully, this overview gave you a basic understanding of these principles to make informed decisions on your purchases.

Looking for products and accessories for your reef aquarium? Browse our catalog here or find a store near you. If you have any questions about salinity or other topics about reef aquariums, click here to contact us.

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