ECG can change in seconds, but water fasting over time can disrupt electrolyte balance. Typically, your sodium and chloride will begin to decrease if all you're drinking is water, as well as other electrolytes like potassium and magnesium. Calcium will leech from the bones to keep levels stable, but an intracellular magnesium deficiency will perpetuate an extracellular magnesium deficiency. Increasing electrolytes is best done slowly, never quickly as it may cause brain swelling if done too fast.
Try adding some sea salt or salt in general to your water, better yet, an electrolyte mix. You don't need a lot, but you need to be able to taste it in your water to the point of you know something is added - not oversaturated.
Can you clarify what type of water fast you are doing or if there is any other pertinent information regarding consumption? Also, what's your reason for doing a water fast? Medical history? Pertinent objective data with subjective opinion helps too.