water softener salt vs potassium chloride

In order to make my water changes I have to add some warm(soft) water
to the cold(hard) water. I know the salt (not sure how much is
present) is not the kind that is naturally in water. Is there any
advantage to my fish to use the more expensive potassium chloride
water softening salt? I hear it's better for your plants and doesn't
leave that whitish salt makes on your soil but what about fish?

TIA

All the best,

Larry
Southern Ontario