I find it hard to believe that you become low on electrolytes after only a few days of water-only fasting.
A few questions for you.
What is your pre-fast diet like?
If it is a plant-based diet of say approx' 80% raw and 20% cooked, and you have adhered to this for several months at least, then a deficiency of electrolytes after only a few days of WF would be nigh on impossible. The only explanation would be a poor or predominantly processed diet, and/or a malabsorption problem.
Are you entirely certain that what you describe as "my heart seems to negatively react" is due to this reason?
Have you been tested for electrolyte levels?
In what way do you mean that your heart reacts negatively?
Electrolytes, as with other nutrients, are conserved during a water-only fast, and much more so than while eating, along with the lowered rate of metabolism that fasting affords, as Natures way of preserving life to the maximum.