Here's what I did:
I floated the bag in the tank and used the algae magnet to hold it in place. I took a 3 gallon jug and placed it above the tank. I put about 3 Big Gulp-sized cups full of water from the tank in the jug, and then dripped water into the bag. Every so often I'd use a turkey baster to remove the water from the bag. And then once the bag was full I emptied half the contents into the bag.
I figure given the small amount of water dripping into the bag floating in the tank, the temp would not be a problem. And since I was doing it slowly, the pH in the bag and the tank would slowly equalize.
I let the jug empty into the bag overnight, of course it overflowed back into the tank. The next morning, I emptied 50% of the water from the bag, started the drip again for another 4 hours. Then I gently placed him on a rock.
I just looked. He's on the move!