Velvet outbreak!

[Aquattro]

Might be too late at this point, but chloroquine phosphate would work in treating the tank if you don't have corals. Not sure where you could get some on short notice though..
Vet can give you a prescription, but that's going to take a while. I sold lots to members on the board, maybe one of them has extra that they can offer? When I got it in my tank, my fish lasted about 4 days tops.