I add while filling. My limited chemistry history tells me that the dechlorinator will neutralize the chlorine almost imediately. The danios I have killed took 6-12 hours to die from chloromine (I only removed the chlorine with some generic drops )
I use my python to clean and remove dirty water.
I have several large trash cans I premix water in.
I remove water from cans with a small $35 pump then refill tank.
The trash cans are on wheels also.
This works for my basement fishroom,would not work well in livingroom...LOL