Are you using buckets to fill the tank? If so, add the appropriate amount of declorinator to the water in the bucket. Just figure out how much is needed for the size bucket you are using.
Some people, if they are using a python, add the declorinator when the start refilling the tank... that is how I do it, and I haven't had problems that I've noticed with that method, although I will admit, one of my fish ended up dead the day after a 25% or more change... I don't know if he got in the water stream, or what might have happened.
With my water garden out back, when I add water, I just add the declorinator and then start adding from the garden hose. I never know how many fish I have in there, so again, I don't know if it has harmed the fish by doing that.