The magnesium will not hurt you unless you take too much at once (it's a pretty effective laxative). We've had a number of people whose arrhythmias have responded very well to magnesium, even allowing them to drop some medicines. Unfortunately, I'm not one of them, so I have to say it doesn't work for everyone. Always work it out with your doctor before you drop a prescription.
Too much of one mineral, such as salt, is generally not good for people who have any heart issues, and there is no benefit from Himalayan salt vs. Celtic salt, vs pink, green, or blue salt. All are sodium chloride, with maybe a tiny trace of something that gives it color. Extra sodium can cause you to retain water. A sports drink (Gatorade, PowerAde, or similar) with a mix of different minerals (sodium, potassium, etc.) at low concentrations may help with electrolytes, and particularly so if you've been sweating (aren't we all this summer?).
Best wishes,