If you want the least amount of risk, use a new sand bed in the 55. Moving old sand beds can kill your fish. It happened to me once.
Here's what you could do (assuming the new tank is going in a different location).
1. Set up the 55 with new sand. Take advantage of having to do a water change on the 125 to give the 125 water to the 55.
2. Since you are using fresh sand and the same water, you can start to move your rocks over at your leisure. The rocks can seed the sand in the new tank by themselves. I find that many of the critters running around the sand also live on and in the rocks.
You can also carefully scoop some (say a cup or two) of the top layer of sand and transfer it to the top of the new sand. A few times, I have taken a shallow container and filled it up with old sand, I set it on the new sand. After a few days, I pull out the container. You can also use this trick if you want to have a little sand bed in the back corner of a barebottom tank.
3. Once you get the new tank set up as you want. Let it run a few days, then transfer the fish over and break down the old tank.
If you're putting the new tank in the same place, you can pretty much do the same thing; but, you'll have to set up a temporary holding tank off to the side.
Best of luck,
Roy
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