Get a hose long enough to reach a sink and faucet! Makes changing the water only time consuming not back breaking.
I use a 50 foot light duty garden hose that i bought just for this. I connect it to my kitchen sink faucet (using a simple adapter available at any hardware store). Then I put the other end in the tank and turn the water on. Let the air run out of the hose. When the bubbles stop, disconnect the hose from the faucet and a siphon will begin (assuming the tank is set-up higher than the sink!). Remove the water to your desired level, vacuuming as necessary. When the tank is sufficiently drained. Hook up the hose to the sink again. Turn the water on and add the dechlorinator DIRECTLY TO THE TANK, FOR THE FULL VOLUME OF WATER IN THE TANK.
I leave my heater running during this process, because it is fully submerged and low in the tank. I then use cold water to slowly refill... the fish seem to love it. It causes them to spawn at times. ;-) Cold water must make them think its a spring thaw or something. lol
Then, remove from tank when full and empty into a bucket, or whatever all the water still in the hose.