They are now all in QT with a bit of cupramine and a little formalin to treat for both brook and velvet.NOOOOOOO!!!!! DON'T MIX CUPRAMINE AND FORMALIN. It makes the Cupramine 10x more toxic and will kill the fish. Cupramine should not be mixed.
I'd bet it is velvet. Velvet is really nasty and can spread fast. A very good test is to do a FW dip and see if anyting falls off. Velvet will fall off in a FW dip and looks like fine white spots/powder. If not then I would think it might be brook. If it is velvet then to only real treatment is copper (like Cupramine) in a seperate QT.