Look at the fins nearest the gills. Can you see specks on them? Ick starts on the skin and spreads to the gills. Velvet starts in the gills and spreads. That's why it kills faster since it starts in the gills where it isn't evident for a while.
When I had velvet some 7 years ago in my then new tank, it killed about 9 fish inside of 2 weeks. The fish that lived was goby, a mandarin, and some chromis I think. Been so long. I did add soome velvelt treatment that made the water turn yellowish for a day or so. The coral, inverts, and remaining fish lived. I can't remember the solution though! Besides, I got lucky I think. I added fish a lot more slowly after that and maintaind my water quality a lot better.
I was new, excited, and adding stuff every 3-5 days. Phosphates and salinity was all over the place. Fish were VERY stressed. Did you do anything similar?????