The danger of potassium and heart attacks is typically only for refeeding after very lengthy fasts and/or people who have preexisting conditions. And it’s made worse by breaking a fast with carbs. A fast of less than a week is really at no risk unless you’re starting out deficient. Still, it’s easy to supplement with Salt and No Salt (potassium chloride) and get enough sodium and potassium to not risk deficiencies during any length of fast. Both of those will also help you feel better during longer fasts.