Congrats on deciding to do a 7 day water fast! Will this be your first or have you fasted before?
It sucks that you had a bad haircut experience. I’ve had a few myself and growing the hair back out again is never fun. Have you had much success in the past using Biotin to speed up hair growth?
And, no, taking it while fasting will not negatively impact your fast in any measurable way. It could possibly upset your stomach since you won’t be eating, but that’s about it. You could try it and see how you feel. You always have the option to pause the biotin until after your fast is over if you choose.