Well, I have read a lot of what people do with pythons. I personally, suck out the water, and replace it first into a bucket, treat it, then put the water in.
I have read where people drop dechlornators into the tank as they put the tap water into it. This doesn't make sense to me. Your fish are still going to get the chlorine this way.
If the product says 3 drops per gallon, how can you accuratly get the chlorine out if it is despersed among the water, instead of all the chlorine being in a bucket in one place.Doesn't seam like it would work this way.
Anyway, I love mine, I have tanks upstairs and downstairs as well, so no more buckets to spill or carry.