Christine, Magnesium can't help as much as drugs.
If you have some serious problems, Magnesium can't help.
Magnesium can help in cases if you have a HR 80-85, to lower it to 75-ish and similar.
Or if you are going through a withdrawal from Beta blockers, then it can help to slightly lower a high HR which you have due to a withdrawal.
But if you have, for example, too fast Heart rate, like 100, 120 and similar while resting, then Magnesium can't help.
Regarding side effects from Bisoprolol, you can ask your doc about some other Beta blockers like Atenolol or Nebivolol.
Or, you can try some other class of drugs, like Calcium channel blockers.
For people who can't handle Beta blockers, if you have a sinus tachycardia, you can try with a new drug called Ivabradine.
So, maybe you should try Atenolol, Nebivolol or Calcium channel blockers.
But, ask for only 20-30 pills, and not for 90-120 and similar.
So, if you won't like them, so that you can switch soon to something else.
Also, there is a chance when you will switch between Bisoprolol and some other Beta blocker, or when you will go from Beta Blocker (Bisoprolol) to Calcium channel blockers that you will suffer a mild withdrawal effect for a few weeks.