Well, the way to figure it out is if you have muscle weakness/fatigue, but not cramping, you may need postassium. But if you have a lot of cramps/spasms (weakness caused by spasm-not generalized weakness) then you need magnesium--and a lot of it. My accupuncturist told me most fibromyalgics are severely depleted in magnesium. She told me to increase my chelated magnesium sufficient to give me 'the squirts' then back off 200mg. It took 2500mg/day to get me to that point. I stayed on 2200mg/day for about 3 months then noticed 'the squirts' returned so I backed off another 200mg. It took about 6 months for my deficiency to be made up. Now I take about 1000mg/day, which is still a lot, but I evidently need it. I also put epsom salts in my bath water every now and then and let my skin absorb the magnesium. I tried the potassium thing, but I couldn't really tell much difference. I have found that when my body does want more potassium I start to crave pistachio nuts (they have more than twice the potassium per ounce than bananas), and pistachios don't interfere with my low carb way of life. Also anyone out there on potassium-sparing blood pressure medication do not need to add potassium supplements to their diets--it could be dangerous.