I just picked up a box on sale. From what I've read people say its fritz salt in a liveaquaria box.I'm using the liveaquaria brand salt without issues. Previously I was on aquaforest
I mainly use IO for my auto water changes but my last box I had to add calcium to my mix because it was low like 375. I had to bring it up to my tanks level which is 450 because it was messing with my dosing.My thoughts,
If your dosing, just use Instant Ocean.
In my last run in the hobby I started with Reef Crystals, but tried Red Sea CP and Aqua Vitro Salinity. I liked and had success with all of them. But the more I dosed, the less I wanted to pay extra for a salt that supposedly had more of what I was dosing anyway. I switched to IO and never looked back.