I feel your pain. I introduced two fish into my system last Boxing day and had my fish get sick with velvet within 2-3 days.
It starts off looking similar to ich but then coats them in a whiteish blanket of velvet. I lost over 50 fish including some of my long time faves. It wiped out a lot of fish but there are at least a dozen that I thought were gonners that have recovered completely. That said, I still have another 3 or 4 fish that were really sick and recovered but still have some cloudiness in their eyes that I am possitive is just scarring from when they were sick because I have added other fish since then with no ill effects. I guess they now just have cateracs. Anyway it took about 3 months to run it's course. If they will continue to eat the stronger ones may have a fighting chance to beat it. I am very sorry for your losses and hope it doesn't continue.