for those that add the water directly to the tank during water change, is it best to add the dechlorinator to the tank before you start filling or after it is done filling? Is here an active time on the chemical. Like, does it only work for a minute once added? If so it would probably be wise to add prime after done filling. If I can add it before I start filling and the prime to continue its job until i finish filling. hmmmm. Anyone know???