Aquarium Hydrometers measure the salinity in a fish tank
by measuring the specific gravity. Specific gravity
is defined as the ration of the weight of one liter of seawater to that of one liter of pure water at a specified temperature.
A new hydrometer will be accurate only to a certain degree, usually measured in PPT's (parts per thousand); if you want more precise accuracy, you should use a Refractometer. Hydrometers will lose their accuracy over time, so you will want to calibrate your hydrometer on a monthly basis. You can determine if your hydrometer is working correctly by testing it with distilled freshwater, which should be 1.000 after correcting for temperature.