From The Aquarium Wiki
A hydrometer is a device used to measure the specific gravity (sg) of water.
It is commonly used to measure the amount of salt in a body of water when setting up brackish or saltwater tanks. You'll want a hydrometer with a specific gravity scale of at least 1.005 to 1.030, salinity of 5 to 40.
Cheaper passive hydrometers are designed to be used at their calibrated temperature. Usually 25°C (77°F) . If you measure at different temperatures you'll have minor errors.
- For more accurate devices, see - Refractometer.