The problem with tap water is you don't know what is in it...true the chlorine can be removed with chemicals or by letting the water sit out but that still does not get rid of any other chemicals that have been added. Nitrate, chloramine, phosphate, fluoride, iron, etc. etc. I won't even drink my own tap water.
water is water that has been filtered through membranes that trap all but the smallest molecules so all that is left (in theory) is pure water. You can buy RO
systems for home use for 100 bucks or so, or you can pick it up at one of those filtered water machines for around 25 cents/gallon. Most machines use activated carbon and reverse osmosis for filtration, just check the labelling on the machine, they usually say how the water is prepared.
If you must use tap water, at least use a dechlorination additive like Amquel, Aquasafe, or Prime. These additives will neutralize chlorine, chloramine, and ammonia making the water safer for aquarium use. The main problem encountered with tap water use is algae blooms from excess phosphates and nitrates.
I usually get 3 gallons of RO
from a machine, add the salt mix, drop in a powerhead and a heater and let the water mix overnight. After mixing check the specific gravity and change out the 3 gallons.