Which magnesium should I take?

I am a cancer survivor since 2006 from a adenocarcinoma vaginal tumor which was treated with 36 radiation treatments. About 5 years later I developed lower intestinal issues diagnosed as radiation enteritis. I also started having bouts of atrial fib that would come and go. I found out that my potassium and magnesium levels were real low and after receiving ivpb of magnesium and potassium and cardiazim 10 mg ivp I would convert back to a sinus rhythm. My B/P runs on the lower scale ( 120’s/60’s) and heart rate in the 60’s. At this present time I am having more frequent bouts of a fib. My B/P doesn’t really change. I usually will take 325 aspirin, magnesium 400 mg, Potassium 20-40 mg and cardizem 30 mg, lie down and rest and it goes away. Digestion issues continue. I have a bowel movement every time I eat something and they are usually normal unless I eat something my gut doesn’t like than I get loose stools which can happen when I take magnesium sometimes. I take the Calm brand. I am a holistic R N and use various modalities to stay healthy. I only take cardizem prn, armor(thyroid) aid, potassium and vitamins digestive enzymes, pre/ probiotics. Would it be possible to speak with you concerning my a fib and digestive issues? I live in southern Calif.