Empty your display tank of
ALL remaining fish (sounds like you did and they're in a QT) Treat the fish with seachem cupramine in QT tank and leave the display fishless for a minimum of 6-8 weeks.
I had velvet go through my tanks a couple of years ago worst disease ever in this hobby.
Here's a link to the thread I did when I had velvet, lots of info in it.
http://www.canreef.com/vbulletin/showthread.php?t=51938