On days I don't eat, I supplement various vitamins and electrolytes to minimize risks of refeeding syndrome[0] and heart problems[1,2]. I don't know how bad the risks are in practice for these for short fasts (I've only gone up to 3 days), but I am sufficiently paranoid that I supplement anyway. Supplementing electrolytes also means I don't get headaches, restless leg syndrome, or (as much) weak feeling in muscles, all of which I used to get when I tried unsupplemented water fasting.
I also try to break the fast with skim milk or yoghurt so that I'm not phosphate depleted (refeeding syndrome again).
For what it's worth, my supplement regime amounts to: a generic A-Z multivitamin, 800mg potassium chloride, 300mg magnesium citrate (it seems to be important that it's citrate for bioavailability reasons), 800mg calcium, and a B-vitamin complex (as I remember it, vitamin B1, or thiamine, is important when refeeding, for reasons I can no longer find).
[0] https://en.wikipedia.org/wiki/Refeeding_syndrome