Some stores run copper in all their fish tanks. You can run copper or you can do a Hyposalinity treatment in QT too.
Are you doing a full 30 day QT on all new livestock?
If so, then I would assume your tank already was infected.
When my 125 broke out with Ich a few years back I didn't lose a single fish. I moved all of them to a 30 gallon tank and did a 7 week Hypo there. It took several days to get the tank set up as I didn't have the tank or equipment laying around.