add dechlor for the entire tank if you are adding the water straight to the tank. otherwise, it will get too diluted and may not dechlorinate the water you add fast enough. personally, I always add my dechlor to the water i am putting in the tank first. i add the amount i need to the bucket I am using and then add the water. that way, the dechlor is mixed through the best it can short of using something to mix it.