FWIW I just did a 5-day fast and took an effervescent tablet for magnesium.

It said "zero calorie" on the front but when I looked at the label each tab actually had like 3 calories.

Didn't seem to break my fast (lost 9 lbs in 5 days), but it depends on how strict you want to be. For example I had several cups of black coffee per day during my fast, which may have added 5-20 calories. Whatever.

If you're going for a strict water fast, electrolytes are fine, just make sure you take them in a solid pill.

For me, the 3 calories in the tablet were useful because they added a citrusy flavor. This allowed me to also mix in iodized salt to meet my potassium and sodium requirements and not taste the salt at all. Downing salt-water is really gross to me.