I use Sun Gems. It's sold as a water softener and made from saltwater in the Netherlands, Antilles. It's dried seawater $ 6 for 60 pounds at Home Depot.
When buying water softener salt for your tanks stay away from the bags that has additives like the ones that has stuff that removes iron from tap water.
I've used the aquarium salt by API they sell at the LFS (but its kinda pricey) and alternatively marine salt such as Instant Ocean I have left over from my saltwater tank to dose my freshwater tank, when needed.