Honestly, it is doubtful that the ich/velvet was isolated to only the Kole tang if you introduced it to your DT. Once introduced to your DT and showing signs, it is my understanding that there are two scenarios that can be at play: 1) If the Kole tang was the most recent fish introduced to the tank and not pre-quarantined and medicated, it may have brought the ich into the DT- and by the time you're seeing the signs, it is highly likely (if not 100% likely) that some of the tomonts (the white cysts on the fish) have already fallen off the fish and will begin to reproduce; or 2) the ich/velvet was already present in the tank, but just wasn't presenting the signs of the parasite on the fish inhabitants, and the Kole tang's immune system became compromised due to some sort of stress (dietary, tankmates, etc.) and allowed the parasite to take hold.
Best case treatment method would be to remove all of the fish from the DT, medicate, and let the DT run fallow (the timeframe I've seen runs between 45 days if the tank is run at 82 degrees or higher to 72 days), but I would check out humble.fish for a more thorough treatment plan. Alternatively, you can try to manage the parasite in your tank by minimizing the stressors that could cause a decline in the fishes immunity (e.g., feed quality and variety of foods, choose the tankmates carefully, etc.).
Good luck!