I got the same problem, all of my fish got ICH. (don't have QT before, and don't know much). That's why I got a hex 5 gallon, for the smaller two fire fish. and the rest in the 20 G. I am thinking about converting my 10g freshwater to a QT too, just in case i need it in the future. i am planning to keep one damsel in the QT at all time to keep it cycle. (may put him in the main tank if no ICH after two month in main tank)