The value of specific gravity, can change overtime if the aquarium water starts to evaporate. According to the rule of thumb, a good specific gravity to maintain for your saltwater aquarium will depend on whether you have fish-only saltwater tank or reef tank. Both setups will require different density and the normal value for fish-only marine tank ranges between 1.020 to 1.023 while reef tank needs higher value that goes between 1.023 to 1.025. Bear in mind also that different saltwater tropical fish will have different requirements and thus the density value has to be adjusted according to the species.
Other than hydrometer, there is also another salinity checker, which is Refractometer. Both equipment are using different concept of measurement in which hydrometer is based on buoyancy while refractometer is based on principle of light refraction through liquids. Both units will also provide fast reading but however, for more accurate measurement, usually experienced fish hobbyist prefers using refractometer. Price is also another determining factor towards your selection because hydrometer usually cost around $20 while refractometer can go as high as three times the price. There is also another reason why sometimes fishkeeper prefer refractometer because unlike hydrometer, refractometer does not lose accuracy overtime.
Nowadays, getting specific gravity reading is quite simple because all a person needs to do is to dip in the hydrometer and the results can be obtained from the gradual marking. For refractometer, this will require a certain skill because you will need to place a drop of aquarium water on the prism and you need to do adjustment by viewing through the optical lens until a clear reading is obtained. Finally, based on the reading given by the hydrometer or refractometer, you can either add in more marine salt if the density value is low or you can add more water to dilute if the density shows higher value above the range that you want.
Other saltwater aquarium supplies: Protein Skimmer