I’m not sure what constitutes fast friendly but I started using this during keto because it has no sugar AND no alternative sweeteners. It has citric acid for some flavor but otherwise seems to be all vitamins. It tastes pretty good and my only complaint is that I have to remix the water frequently.
Anything with a sweet flavor will spike insulin levels, even if it’s “0 calorie, 0 sugar”. So you’d be looking at a dirty fast. I’d recommend drinking plain water with pink Himalayan salt during fasting window, and drinking your flavored electrolyte mix of choice during eating window.