Doctors say my Magnesium is low at 1.3. The said to double my dose from 400mg once a day to twice a day? Anyone else struggling with low magnesium?

I am from Michigan but spent a significant amount of my adult living in the Chicago suburbs. I am retired and spend a significant amount of time on the water and love to scuba dive. In 2015 I had a heart transplant that we believe was caused by a hereditary trait from my mother. I am focus and taking the best care of my new heart since that day. I have change a number of my bad habits which included a poor diet and lack of exercise. I am always looking at ways to learn and grow in my health choices. My doctors are amazed at how well I have recovered and am living with my recycled parts for a guy who is 68 years old!