I don't see that much of a shift killing off Clowns, but I have to agree with Greech here. Don't see anything else that would cause it.
Unfortunately anything else we won't be able to narrow down very easily. There is a chance a disease slipped through the QT process, but if it is a disease that shows no symptoms (entirely possible) then there is nothing to diagnose. Same with something living in the tank, could be some unknown hitchhiker but without any sign of injury there is nothing to identify. For now I guess you can only write it off as salinity issues, but I'd keep a close eye on things since there may well be something else going on that just isn't easily seen.