Salt for an Aquarium

I have several saltwater aquariums , requiring partial water changes .without the access to pure ocean water , mixing my own is somewhat of a challenge at times .I have searched for information and I hope to find out if , pure sea salt that is sold in large bags for water conditioners I believe . it says on the bag that it is harvested from drying seawater with nothing subtracted or being added to it .is this the same as the salt being sold at the pet shops for use in aquariums ? also when ocean water evaporates , can other substances also escape with the water vapor ? could you take the powder remaining after the seawater dries and add it to pure water to return it to seawater ? thank you for any time that is shared .