Fasting is different than ketonic diet though. With fasting you also lose constantly salts/minerals/vitamins. Especially salt deficit will make on tired.
What do you think about drinking water with electrolytes or those electrolyte “salt”rich tablets that have magnesium, sodium, calcium, etc? (While fasting of course)
It's important while fasting. Your body needs electrolytes or you will have all sorts of health issues. Potassium is the most important one, followed by sodium. Every cell in your body has a sodium/potassium pump, which works to keep potassium inside your cells and sodium outside (creating a battery essentially). About 30% of all energy in your body is used just to maintain this pump.
Anyhow, potassium is the big one... many people don't get enough from their diet even while not fasting. And unfortunately supplements give very minuscule amounts of potassium (<=2% of RDI), supposedly because rapid absorption of large amounts of it can cause heart arrhythmias/attacks.