You *can* use tap water along with a dechlorinator but it is recommended that you use RO
water instead. This is because using RO
water will remove >90% of the nutrients that undesirable algae love.
So as long as you don't mind putting up with continuous algae problems, tap water is fine.
I will note that some people's tap water is so clean that they don't have to use filtered water. But this is normally few and far between. The majority of city water has all kinds of chemicals in it that will fuel algae blooms.