It could be possible that the other fish won't. Clownfish can hold out for sometime for example. Unfortunately, the other possibility is a string of quick fish death. Once you're confident you have velvet pull the trigger and treat them all.I see...
I think I have read that CP has a negative impact on the fish immune system. Should I do a transfer to a clean tank after a week (to cover both ich and velvet)?
If this is velvet is there any chance that the other fish in the tank don't display any kind of symptoms over the next few days?