I use baking soda to increase alkalinity and replenish bicarbonates.
According to the label, Reef Complete contains includes some magnesium (though probably not at the quantity for the purposes of increasing mg levels).
I've actually never tested for magnesium (probably something I should do), but I feel that the magnesium in my salt mix (combined with the periodicity of my water changes) is keeping mg levels where they should be. Indeed, a test would confirm this.
With that said, I'm not sure I'd recognize indications of a magnesium deficiency in a reef tank.