This may depend on where the water was collected. In my experience, seawater collected in nutrient rich areas and kept in the dark for one month was rapidly toxic to small fish. Some British species are euryhaline and will enter estauries where fresh water can rapidly reduce salinity levels. A guide to low salinity tolerance can be gained by observations in the wild, and by examining the distribution maps of species found in the Baltic. It is important to keep ths salinity in aquaria within narrow bounds.
Salts tend to leave the water in the aquarium and accumulate on the cover glass.
If the salinity falls too low it should be adjusted at the regular monthly water changes. A Hydrometer is an essential item of equipment for measuring the specific gravity, and hence calculating the salinity of the water. There are 2 types available. If the bulbous hydrometer is used, it should be placed in the aquarium water when the filtration is turned off, and the reading taken at the water surface level shown in the diagram. The ability of fish to tolerate changes in salinity varies with different species.