Some people drink black coffee or tea, and some use a sweetener like stevia, and some people only drink water during their fasting time.
It seems that each person has to decide what works for them, because I have seem some people say that they do fine, drinking things like diet pop (soda) when fasting, and others said it didn’t work for them.
I mostly drink water with ice and a little lemon in the summer, and something hot like coffee or tea in the cold weather.