Im never buying IO or RC salt again. I use to use Coralife salt all the time, started buying IO cause it was cheaper, soon there after I started having tank problems. 6 months later everything was out of whack. I went back to coralife salt, a couple of water changes later everything is perfect.
Check this out and compare the salts. Coralife is the best all around salt out of all they tested.
Reef Salt Analysis
Here is where another guy tested them all for Ca, DKH and MG.
A General Guide to Salt Mixes