30 day fast

I am getting ready for a 30 day fast. My first one was 28 days. My second was only about 10 days because I passed kidney stones and decided to stop. I always get heart palpatations. I presume my potassium gets low. Do I need a supplement that contains K, Mg and Ca to do the fast safely. I can't find any supplement with more than 3% of the daily recommendation. What suggestions do you have? Thanks.