Salinity / Specific Gravity
Salt levels and salinity can sometimes be a little confusing. Salinity is the amount of dissolved salt in the water. Pure water has a salinity of 0ppt (parts per thousand) whilst seawater varies around 35ppt. This equates to 35 grams of salt per litre (1 litre of water = 1000g, so 35grams is 35 parts per thousand)
It is not easy to test for salinity directly, but we can test for Specific Gravity (S.G.) which is a measure of the density of water. Dissolving salt in water increases its density, which can be measured as Specific Gravity. However, specific gravity is affected by temperature, this is because as water warms up it expands, making its density lower, but keeping the same amount of salt.
Using the chart below you can work out what the actual salinity of water is based on your specific gravity and temperature. You can also work out what your specific gravity should be at a certain temperature if you are keeping your water at around 35ppt. To aid with this we have highlighted the range of 34ppt-36ppt in green.
NOTE: Most hydrometers are calibrated to work at certain temperatures, although they rarely state what temperature. For the most accurate readings use a refractometer instead, which are easier, more accurate, and almost always calibrated for 20degrees or have automatic correction.