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A refractometer is arguably the best and easiest method of determining the salinity - the most fundamental of water parameters in the marine aquarium. It measures the degree to which light bends (or refracts) as it passes through a sample of water. The degree of light refraction is proportional to the level of dissolved solids in the water sample, which in the case of salt water means the salinity of the sample.

Refractometers typically available to the hobbyist are small hand-held units. Several drops of water are placed on a glass panel, and the unit is held up to a light source and the hobbyist reads the salinity level on a scale through an optical eyepiece.

A major benefit of a refractometer is that it requires only a few drops of water to obtain an accurate salinity reading, which makes it invaluable in effectively acclimating new livestock from transportation bags to the aquarium. Alternative salinity measurement devices such as floating glass hydrometers or plastic swing-arm hydrometers require a significantly larger water sample. Most refractometers are self-correcting for temperature effects and are quick and easy to use. It is recommended to allow some time for the water sample and refractometer to equalize to the same temperature before taking a reading.

It is highly recommended that this unit is the first piece of water testing equipment that should be purchased by the new hobbyist.


A refractometer should be calibrated prior to first use using pure water, distilled or reverse osmosis (RO) water. Distilled, deionised (DI) or good quality RO/DI water will have a specific gravity of 0.000 or salinity of 0 ppt. Tap water from the vast majority of locations will also show a specific gravity 1.000, there is insufficient dissolved material to increase it to a detectable level.

For calibration at 35 ppt salinity / 1.0264 specific gravity (that of NSW) a reference solution can be made from water and table salt (sodium chloride, NaCl) using one of the following measures (scales used should be accurate to at least half a gram):

  • 14.2g salt and 375.0g of water
  • 14.6g salt and 385.4g of water
  • 20.0g salt and 528.0g of water
  • 36.5g salt and 963.5g of water