I'm experimenting with fasting too. I did five days last week and was shocked at how easy it was. Now I'm on Day 4 of another one and really, the only problem is mild boredom.
You really realize how much we eat and drink for pleasure, and how little we actually need.
I drink plenty of water when I fast because, rightly or wrongly, I think my system is flushing out the wastes of whatever it is breaking down and eating.
I always feel wonderful the next morning...and light on my feet.;-)
Best of luck with your keto and fasting.