You don't necessarily have to, but it ensures that all of the chlorine is dealt with quickly, if you only dose for the water added you have to wait for the dechlorinator to come into contact with the chlorine for the reaction to occur, if you increase the concentration of the dechlorinator the probability of the chlorine coming into contact with it is higher so it is neutralised quicker.
I see what you are saying... That makes sense. I usually put the dechlorinator in the bottom of the bucket first, and then fill the bucket from there. (plus a little extra), I guess it's along the same line of thinking. I fill the bucket up with quite a bit of water pressure to mix it up pretty well the entire time, and then by the time it reaches the tank, it has had quite a bit of time for it to dechlorinate.
Thanks for the information.
