Water will evaporate from your aquarium and need to be replaced on a regular basis. When adding replacement water only use prefiltered water i.e. reverse osmosis (RO) water or conditioned freshwater. Salt in the water does not evaporate. It is a good idea to check your specific gravity with a hydrometer every so often to ensure the salt content is at the desired level. If the water has a low specific gravity ( the amount of salt in the water) instead of adding freshwater to top off your aquarium, add premix saltwater to raise the salt content.
Most hobbyist use plastic aquarium hydrometer with the swing arms which give an accurate reading on specific gravity. But many hobbyist neglect to clean them. The pointer or swing arm will accumulate deposits over time. Rinse the hydrometer in freshwater after each use. For a thorough cleaning soak the entire hydrometer in vinegar (mild acid) for about 30 mins. and rinse. This will keep your hydrometer giving you an accurate reading for years.