I use the system from Petco - the ease/speed of filling alone is worth it to me. As said above, I just treat the tank before starting the water running back in. Instead of calculating/treating the amount in each bucket, just add enough conditioner for the volume of the tank.
I will say, I am not impressed with the durability of the plastic adapter to connect to the sink. I have been overly careful with it as far as tightening, adjusting, not bumping it, etc and there are still already three cracks in variouos places - the plactic is NOT durable at all (which I expected and is why I was being so overly careful to begin with). None of them are to the point of making it usless yet, but will eventually spread as I use it - I've only had it a few months as it is. That being said, I'm heading to the hardware store in the next couple of weeks and going to just pick up some more durable hardware to fasion my own connector and there will be no problem. In hindsight, could have saved myself some $$$ by doing that to begin with and buying the needed length of plastick tubing to reach sink to tanks.