It will still do a little cycling. You need bacteria to build up in the new sand as well as the on parts of the glass, and throughout the plumbing.
Especially since you just added new water to the system.
That can help. Take a couple cups from the main tank, and swirl it into the new tank. You don't need to take any from the prop tank, unless you just feel like it. Keep in mind, new lighting will also cause algae blooms.