I have done several water fasts, ranging from 4 to 35 days, and only once have I experienced what I believe to be an electrolyte imbalance. This happened after just 7 days of fasting, while doing a 25 miles walk in very hot weather. Not a very good idea, I admit, but this walk is a yearly big event in my town and before this I had never had any problems with moderate activity while fasting, certainly not in the earlier stages. Also, when I started out in the morning, I had no idea that it was going to become that hot. I had problems with palpitations, a wuzzy feeling in my head and, what I found the most worrying, although being very thirsty vomiting most of the water that I drank. I also had instances where my arm and leg muscles felt sort of paralysed for upto 3 days afterwoods.
However, I believe that under normal conditions, when it's not too hot and you're not working out too heavily, an electrolyte imbalance is very improbable. My advice would also be to stock up on potassium beforehand, for example by drinking a lot of potassium broth. However, if you do decide to use the electrolyte supplements, I would be very interested to hear about your experiences. I do believe that you would still be Water Fasting
when you do this because, unless you are drinking distilled water, the water that you drink is bound to contain some minerals anyway.