I agree on magnesium. I think most people are deficient in magnesium. It's hard to measure, because it's intracellular magnesium levels that count (not blood level). There is a test for intracellular mag levels, I think it's called an "EXA test," but it's not usually done. If you are mag deficient, your body will lose it first at the intracellular level, then the blood level. So even if your blood tests fine, you can still be deficient.
For magnesium supplements, the most commonly available is is magnesium oxide. That also is the form that is least absorbed by the body (something like 6% is absorbed).
The chelated forms of magnesium are much better absorbed. These are the ones such as magnesium glycinate, taurate, malate, citrate, etc.
I like this one:
https://www.amazon.com/gp/product/B01GW4V7MC/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
IMO this is a good supplement for anyone.
I also have a glass of low sodium V8 juice every morning, which has around 1500mg of potassium. US RDA for potassium is around 3500-4000mg. Take a look at your diet and see how much potassium you are getting every day!