Velvet sometimes affects fish without outward spots or symptoms. The other reality is that by the time you see it on the fish, it is often too late.
It may well be ammonia that is killing your fish. If that tank was not fully cycled, daily large water changes will be needed to keep ammonia down or that will end them. Has anyone been doing water changes and replacement medicine dosing?
The tank was not even running 24 hours yet at the point they died, so it couldn't have been ammonia poisoning. Also, this is a 40 gallon tank, so there is sufficient water volume to make it a few days.
I've read @Humblefish say not to water change for 30 days. How can you treat copper, not water change, and control ammonia at the same time?