Magnesium chloride or oxide

by Johnny
(Arizona USA)

What is the difference in Magnesium chloride and Magnesium oxide. I have been taking the oxide form of Magnesium and why should I be using M-chloride instead? I have CHF and Atrial Fibrillation. I also have a thickening of the lower lobe, the Dr's don't know if I was born with that or if it has developed over the years. I am 71 I am somewhat active but I get real tired starting in the early afternoon.