From Diamond Crystal's FAQ on water softening salts:
"As with food considerations, water softening salts are not intended for human or animal feeding. The particle size is inappropriate for small animals. In addition, water softening salt may have additives that are inappropriate for animal feeds. "
I'm not saying the kind you have is going to kill your fish, but the terms they use would make me a little nervous to use it.
Instead of that stuff, I would use their Diamond Crystal® Pool Salts: Per their website, it's "Over 99% Pure Salt....Natural: Our pool salt is made with salt, and salt only."
It sounds like the water softening salts may have some additives, but their pool salt does not, which makes the latter the better choice.
I used Leslie's pool supply salt (sold in 40lbs bags for $8) to treat my tank during an Ich outbreak and my fish all pulled through just fine. I dosed up to 3 Tablespoons per 5 gallons.