Marine Aquarium Salinity

I often get asked questions about Marine Aquarium Salinity.

So lets start at the bottom and work our way up.  

How do we measure specific gravity

Measuring salinity is a quick process using a Hydrometer orRefractometer.  


Refractometer Measuring Specific Gravity

Twenty five years ago a hyrdometer would have been used by anyone with a salt water aquarium but these are often give false readings. The swing-arm type of hydrometer is very cheap and easy to operate but is prone to degrading and often becoming so inaccurate that I recommend to not even bother buying one to save money.  

Remember how important the measurement of salinity is and that a hydrometer will measure specific gravity (relative density) of liquids and display the ratio of the density of the liquid to the density of water. This is something that should be given a large amount of importance as it can be the difference to success or not.  

Measuring Specific Gravity
Measuring Specific Gravity is easy with a Refractometer you simply put a small amount of water onto the glass chamber and look through the device at a light and you will see the blue line appear at the current salinity level. The major benefit of the refractometer over the hydrometer is accuracy. The lat point to make is that the refractometer is so much more accurate than the hydrometer so I would recommend the first thing you buy is a Refractometer. A very important things to note is that when calibrating your refractometer you should do this with RO water or tap water at 20 degrees.


A saltwater fish tank (marine aquarium) will be the home to marine animals from tropical climates. The temperature of these tropical areas around the world will vary and can be between 24 and 28 degrees Celsius (76 – 82 °F).  

Great Barrier Reef

Tropical reef tanks and also fish only aquariums will require you to match the temperature of the home environment  of the waters they come from. The most important thing to note is that keeping a temperature stable is one of the most important jobs when setting up. 

Temperature effects salinity so if you get everything right at 24 degrees and then over the course of the day the lights and pumps and ambient temperature raise the aquarium temperature to 28 degrees the salinity will now be totally incorrect.  

If you measure your salinity at 1.023 and the aquarium temperature is at 25 degrees (77°F), the actual specific gravity of your aquarium water is 1.025. Note that minor fluctuations can be the cause of problems for your aquarium inhabitants, if you have leather corals for example that suddenly look like they are dying this can be a quick sign that your salinity is wrong.  


Salinity is the measurement of the total amount of dissolved salt in seawater. This is measured in parts per thousand (ppt) and the average salinity of the ocean is around 34 to 37 ppt.  

Marine aquariums will be measured in specific gravity. Specific gravity may be defined as the ratio of the density of the liquid to the density of water and the density of liquid varies with temperature and  so does that of specific gravity.  

The specific gravity of seawater at 35 ppt is 1.026. I recommend that for your marine aquarium you aim for the lowest point being 1.021 and the highest 1.026.  

To help get the correct salinity no matter what the temperature is use the chart below as a reference:-  

Salinity Chart

I hope that this helps you understand how to setup your salinity for your marine aquarium.

Clown Fish

This entry was posted on Saturday, December 18th, 2010 at 9:21 am and is filed under Salt Mixing, Water Change. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.