I've never had a problem using tap water, but some of the additives in it concern me. Chlorine will evaporate out, but there will still be residual levels, although tiny. Added fluoride is good for humans, but not good for snakes. And it won't evaporate out like chlorine (if I remember my chemistry, and I am a chemist, BTW). So, IMO, tap water will work, but bottled drinking water is the best choice.