Sorry for your troubles. At least your on the right path now. Might be tough to figure out what fish brought that in to your tank, but i would think about your purchases & evaluate where your buying from. It can come through anywhere but some stores certainly are better at keeping it at bay than others. I definitely know of at least one local place i can purchase velvet any given day.
Also of course future quarantining will keep it out.
Also of course future quarantining will keep it out.