Hydrometers, the plastic bodied specific gravity measuring devices with the swing arm, are terrible pieces of aquarium test equipment. They are inaccurate and cannot provide a good enough number that aquarists can rely on for water changes or simple spot checking. The reason hydrometers are no good is due to the fact that the swing arm can be easily influenced by stray bubbles and they do not compensate for temperature well. As an alternative, use a refractometer. They are far better at measuring salinity (and specific gravity) and do compensate for temperature differences.