Specific gravity (SG)

Specific gravity is an alternative way to measure salinity. Indicates the ratio between the density of salt water and that of pure water.

Detailed explanation

Specific Gravity is the traditional method for measuring salt in sea water. A value of 1,000 corresponds to pure water. Natural seawater has SG about 1.025.

The conversion between SG and PSU (salinity) depends on the water temperature at the time of measurement. At 25°C: SG 1.024 corresponds to approximately 33 PSU. SG 1.026 corresponds to approximately 35 PSU.

The optical refractometer is the most reliable instrument. It costs 15-30 EUR and does not require batteries. All you need is a drop of water on the prism, you look at it against the light and read the scale.

Pay attention to calibration. The refractometer must be calibrated with reference solution at 35 PSU, not with distilled water. Calibrating with distilled water introduces a systematic error of approximately 1-1.5 PSU.

The float hydrometer (hydrometer) is inaccurate and should be avoided. Air bubbles adhere to the float, distorting the reading.

In brackish aquariums the SG depends on the species: from 1.005 (low salinity) to 1.015 (medium salinity). Some fish such as Monodactylus live well at full sea salinity, others such as Brachygobius prefer very low salinities.

For freshwater aquariums the SG is 1,000 by definition.

Reference values by aquarium type

How it is measured

Optical refractometer calibrated with reference solution at 35 PSU. Read at room temperature (20-25°C) for maximum accuracy.

What happens when it is too low

Osmotic stress for fish and marine invertebrates. Retreating corals.

What happens when it is too high

Reverse osmotic stress. Cellular dehydration. Shrimps and soft corals are particularly sensitive.

Related terms