Instant Ocean is notorious for having low Calcium and Magnesium. Also, depending upon how much salt you have, the lower the Salt Gravity, the lower the Magnesium and Calcium will be. Usually the ideal number for reef tanks is at 1.026, but a couple more or less is within a safe range. That being said, newly mixed salt water with any salt will have different readings at 1.025 and 1.026.