I used tap water back in the 80's for a fish-only setup. Instead of dechlorinator, I let the water sit out for a few days before mixing it.
However, that was in suburban/rural Ohio and not in an area with known BTEX (benzene, toluene, ethylbenzene and xylene) contamination in the groundwater. Granted, most volatile and semi-volatile compounds will gas off in a few days if you let it sit out. It is the phosphates, silicates and fluorides you will need to watch out for. Your live rock might get awful hairy.