All fish dieing

[gregzz4]

Another possibility is Brooklynella. Even though it's known as Clownfish disease, it affects all. Some it kills and some are only carriers.
Brook kills just as fast as Velvet.

I bought some fish last year and the 4 of 10 that survived through a QT period of 3 weeks infected my DT fish. 1/2 my DT fish died within 3 days, including 2 of the 4 that were introduced.
I had to remove and treat the survivors with CP.
The carrier fish showed no signs so it was impossible to know it would happen.
It was my own fault as I panicked when the QT fish were dying and I thought they needed more room than my QT provided.

If you ever need to house fish again during a move, I'd suggest finding someone with a spare tank who can house them completely independent for you.