The lion had ich on it before it got to your tank. The salinity at the LFS
was probably lower than yours and you didn't see the ich until you put the fish in the higher salinity.
The only way to be sure is to move all fish to the QT and treat all of them with hyposalinity. leave the tank fallow for a period of weeks. I think 6 or so, but I'm sure someone will chime in and give you a better idea.