lost all fish!!

[naesco]

I take it you have a corner upright.
Your tank was never suitable for one tang let alone 5. I it obvious that you have a love for tangs but wait until you get a large tank (6 foot to house them)

I doubt ich or velvet killed your fish.

The ich surfaced when the existing fish became even more stressed with the addition of the naso tang
IMO your fish suffocated to death for lack of oxygen f you have ruled out a extremely high heat spike.