The basic rule of thumb is: if you're losing water, you need salt (sodium). If you're retaining water, you need potassium.
Very interesting. I was going to start a thread here asking about retaining water. I've been retaining water since I got sick. It's related to my PMS bloating but is much more extreme than before I was sick and extends to more than just my belly, but also my face, legs, etc. I did not have this problem before CFS or during CFS remissions. Is it safe to take potassium supplements?