Same thing with Home Cheapo, they have water softener salt, and they have "pool" salt for $3 a bag more.
Ultimately, salt is salt, as long as it's Sodium Chloride. You can look at the MSDS (Material Safety Data Sheet) for any of these products and see what is in them, as long as it's not a proprietary trade secret. (and salt isn't a trade secret, lol).
Morton has two varieties of water softener salt that I saw, the blue label and the yellow label. When you look at the MSDS, it shows the blue label product is 100% Sodium Chloride. The yellow label product includes Sodium Hexametaphospate, too. So I chose the 100% NaCl variety and it's working fine.
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