I use Instant Ocean but when it is gone I will be buying Reef Crystals from then on out. I.O. just doesn't have the Ca that it should, I think when I tested a new batch of it I found the Ca to be 360ppm
Yes, IO has always run on the low calcium side.
I add calcium and magnesium to it. IMO, that makes a better (and less expensive) salt than Reef Crystals for my purposes. I neither want nor know the identity of what the vitamins are that they add, and I want my mix to be stable for weeks in a storage container (anything with organics like vitamins may not be stable and may encourage bacterial growth and metabolism of those vitamins to who knows what).