Moving a Large fish-tank, how to keep fish safe DURING the process?

I can easily drain and move the tank itself. along with the cabinet it goes on. However, I need to place the fish in something to transport them to their new location ( down a flight of stairs) I was thinking of using a food-cooler since it is big enough,can hold water, and has handles. Along with the added benefit of a closing lid to keep fish from jumping out- or my cats from getting a live snack.

Do I need to wash out the cooler with a special kind of soap or other method so the fish will be OK? OR- do I need another approach altogether?

I could really use some technically-sound advice so my pets survive my re-decorating.