I think if it's your first fish for the display, and looks healthy, I'd just put it in the main tank and hold off adding any new fish to the display for several weeks (and QT those before adding). Lionfish are rarely diseased or parasitized if they look healthy and are eating at the LFS.
The way I usually set up my QT is to have a bare bottom tank with cured live rock and a power filter (no carbon). I usually have some macro in the tank too. Water parameters are stable. I put the fish in there to observe health. I do NOT treat with medication or lower specific gravity unless the fish shows signs of illness. If I need to treat, I remove the live rock and any macro to a tub of salt water, treat as needed, and do daily 30-50 percent water changes to keep water quality up. I get medications from national fish pharmacy in arizona. They have a good web site for diagnosing disease and parasites.
I think with a new fish, it's really important to minimize stress, and get them eating well, more than anything. QT lets you observe the fish for disease, make sure its eating, and keep it away from any tankmates that might pick on it. Most of the time, the fish I buy do fine in a 2-4 week QT and never require medication or hypo.
This isn't to say QT isn't essential. All it takes is one sick fish to crash your whole tank. This happened to me in 2005, and I lost fish I had for 3-7+ years.