wondering...

Do you mean can you add the dechlor directly to the tank with fish in it? If so, yes you can. Just be careful about the dosing, obviously.

If that's not what you're asking, I'm confused!

Usually, yes. Is there a specific reason you are asking? Certain dechloraminators can be does directly to the tank in case of ammonia problems, for instance. I use a Python to drain my tank and refill it. I usually add some Prime before I start the refill, then add some more directly to the steram of incoming water.