If it is truly chlorine in the tap water, aerating the water will remove the chlorine. If it's treated with chloramine, that's a different story altogether (takes much longer for it to break down). I would think that all their tanks are tied together and a water change would be done for multiple tanks at once in the back. Or are they doing individual water changes on each tank?
Regardless of what the store is doing, YOU should consider a RODI
unit and aging the water mix overnight (24 hours is better) with a heater and a power head to stabilize the mix before doing a pwc