Observation QT's at normal or slightly reduced salinity are only good for making sure that any immediate threats to the fish are known before adding them to your more expensive stocked tank. This would be anything that pops up to visibility by your eye (whether marks on the fish or just general health symptoms of the fish). Anything that is missed would just get carried over to the display tank.
To truly make sure you don't infect your main tank you would want to do one step further in QT which is to either run the QT at 1.009 or so salinity for 6-8 weeks (without adding new entrances in the mean time), or running copper in it. Even with those options there are still some nasties that could carry over, but you are taking both an observation and parasite ridding step using those QT methods. There are great write-ups elsewhere on the forum that tell the pros/cons of each QT method. But make sure to start it off right from the beginning or else can haunt you later.
