Velvet kills quick, so that could be the culprit, but there are other things out there too. Did it look like it had a thin layer of stuff on it? Hard to describe, but kinda like someone rolled the fish in a powdery substance. How do the clowns look?
The thing to consider now, is that regardless of what it was, if something was in the tank, it is still in there. If you add new fish, they could come in healthy and end up dying from catching whatever it is. Leaving the tank fallow for about 8 weeks allows whatever it was to die off by not having a host to keep the life cycle going. Its no fun having an empty tank, but it clears up the problem. I am just getting my 75g back up and running after a Velvet outbreak earlier this year.