I wouldn't recommend doing that in small tanks, the chlorine could kill the beneficial bacteria.
If you have a way to drain your old water on the same floor as your tanks (think laundry tub) and you have a clear, non carpeted pathway, I would go to Home Depot and get a Rubbermaid 44 gallon Brute garbage can along with the wheels (this is a disc that screws onto the bottom of the can). Then get a submersible pump, like a Mag 12 or 18, and attach about 8' of hose to the discharge side. You can now rapidly drain your tanks into the garbage can with the pump, and drain the water out of the can into the tub without carrying buckets.
You can then refill your can with fresh water by either attaching a short garden hose to the laundry tub spigot, or have the water running into a bucket and again use the submersible pump to fill the garbage can. This will also allow you to dechlorinate the water prior to pumping back into the tank.