Actually I never said to QT the fish, I said be prepared to. Unless a parasitic problem, QTing the fish in an uncycled tank would just make matters worse. If a well cycled QT was available I may have spoke differently. Nikki was right though, if the fish was able to be placed in an appropriate QT and the salinity lowered, it would have been a great benefit. Had the fish's injury been beyond just the tail ends, the tang would have had a extreme time of trying to maintain osmotic balance.
There are many advantages to leaving a fish in the main tank, especially in less severe circumstances. In regards to popeye, this is often a misdiagnosed problem. Most eye problems tend to be from injury. If only one eye has a slight buldge, it's not popeye and leaving the fish is actually the best choice. Plenty of water changes with well aged/aerated SW along with heavily fortified feedings does the trick just nicely. If true popeye where both eyes are affected, buldging and cloudy; QT with antibiotics is almost a certainty. Typically I will suggest the previous route first but more often than not it will need treament.
Cheers
Steve