people who say to use supplements with it or its okay to drink coffee just don't understand the point of fasting. the idea behind fasting is to FAST FROM EVERYTHING. a water fast you would drink water during a fast, you can add some salt and electrolytes to your water if you are having a tough time hydrating
if you add vitamins to a fast that isn't fasting. that is not eating food and taking vitamins. not the same thing. you won't get the full autophagy and anti aging benefits which is a damn shame and a waste of your fast. but many guys just don't get that part, they only care about fat loss which is not even 1% of the benefits of fasting.
having said that above there are different variations of fasting, some guys will just have bone broth the entire fast, some guys might drink pure coconut water, and some guys will still have coffee/tea. will you get SOME benefits from this? yes. will you get ALL THE BENEFITS? not even close. you won't fully shut down your digestion system for example if you are still taking in nutrients so if you want to do it 100% then just water.
i have many tips on my podcast over on evo, check that out or pm me for more info