Marine fish and corals are very sensitive animals and it is important that you buy a quality salt mix. Quality salt mixes will provide all the trace elements in the correct quantities required for your fish and corals.
Salt poured directly into the aquarium can seriously harm its inhabitants so always mix marine salt in a bucket first, then allow it to dissolve thoroughly, preferably the day before its required use. Once mixed you should test the salinity with a hydrometer and make any final adjustments by either adding more salt or water. If specific gravity is too low, add more Marine Salt. If too high, add more dechlorinated water.
When measuring the specific gravity or salinity of the marine water it should be of a similar temperature to your aquarium. Recommended range is 1.020 – 1.023 at 24 C (75 F) but this may vary dependent on the origin of the species kept.