there are different ways to fast depending on your particular goal... to heal your body you shouldn't take any supps at all, including electroloytes.. you also should not exercise or weight train. this is a hard thing to do but it is similar to an animal in the wild who is injured who finds a den and just lays there for days
the concept of fasting is lost in the meathead community, it is all about stressing out your body and forcing it to detox and allowing it the freedom to focus on repair/rest. so taking any sort of supplement makes the fast less effective if your goal is healing the body