There are different opinions on this. Some will say nothing but water, period. But many fasting experts agree that tea, especially green tea, without sweetener is OK and some even say it is beneficial. I am in the camp that it does no harm, so if it helps you stick to your fast or makes the fast go by easier, go for it!

I personally have green tea almost every time I am water fasting. Flavored teas break the monotony. It helps me a lot.