Lemons contain both magnesium and potassium. The main concern for a mineral imbalance would be from loosing too much fluid through the SWF. This can be helped by only doing the Salt-Water-Flush every other day if you find yourself losing too much fluid. I no longer do the Salt-Water-Flush at all because of blood pressure concerns and still have good cleanses.
If you feel that potassium loss will be a major issue for you then add one or two tablespoons of raw apple cider vinegar to a couple of your lemonade drinks. This will give you a rich source of potassium and not hurt the cleanse at all.
Even water fasters do not suffer from too much potassium loss in short to moderate fasts. I would not worry unless you plan to go over 21 days.