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What is Salinity?

 Saltwater is made up of Sodium, Chloride, Calcium, Magnesium and Sulfate. 

When testing for salinity we are measuring the dissolved salts. This is measured in two ways generally; ppt (parts per thousand) or sg (specific gravity).

Natural seawater sits around 35 ppt or 1.026 sg, this means that for every kilogram of water there is 35 grams of salt. 

With the ocean being so large, salinity and other elements remain relatively stable.

Stability is the goal we strive for in our tanks.  

We highly recommend an ATO (auto-top off), to help keep salinity stable. Fresh water will be evaporated leaving behind salt so it is important to replace with Fresh RODI water as it does, otherwise it can dramatically change the salinity levels harming your tanks inhabitants. 

We recommend testing at least 2-3 times a week as it will save you from a very common mistake!

Our Recommendations



Top Recommendation

  1. Can be Callibrated easily

  2. Most Affordable

  3. Easy for Quick basic measurements

Digital Refractometer

digital refractometer.jpg

Easiest to read

  1. Easy to read

  2. High resolution allows for more accurate reading

  3. Hanna's digital refractometer also has temperature reading as well

  4. Can be callibrated

Callibration Fluid

Important to keep equipmment testing accurately

While the cheapest way to test salinity is a hydrometer, it is NOT recommended

by us. It has been inaccurate and can cause unexpected salinity swings. This is very important especially if you're choosing to have corals.  

How to Read and Calibrate a Refractometer


Once Saltwater is mixed and at room temperature, or if using a sample from your tank, place water droplets under the daylight plate and put the glass flap back down on sample and hold to bright white light. You can adjust the focus by rotating the focus adjustment near the eyepiece. Your water sample should look like the picture above on the left. Where the blue meets the white, the refractometer

is reading 1.025 sg (left side) or 35 ppt (right side).

When calibrating, use calibration fluid instead of saltwater under the glass flap and the fluid should read exactly Zero Salinity/1.000 specific gravity. If it is reading slightly above or below this line, use screwdriver to adjust the calibration screw under the rubber cap until it is at the zero line and redo the test to verify its accuracy. 

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