Agree about the iodine... you probably don't need it. But you *may* need the calcium and alkalinity. It all depends on what is in your tank that is using it. If through testing you find that your calcium/alkalinity levels do not stay within acceptable levels (say 380-420ppm calcium and 8-12 dKh
alkalinity) in the time between water changes, then you'll need to add cal/alk
It really all depends on what salt mix you're using, how big the water changes are, and how much coral/coraline algae you have that's using it up.