Quarantining fish if suspect tank already has ich

[Aquattro]

Personally, I don't quarantine fish, nor would I ever. My existing fish are fat and healthy, and they can manage their own health. I expect the new addition of a Tang would result in some ich, but it's to be expected and my tank provides the right atmosphere to reduce stress (clean, lots of hiding spots, etc).
My last tang was a large RS Sailfin. He looked salted by the time I got him home. Fed garlic on nori, 30 hours later, no sign of ich. It's been a couple of months now, everyone is fine.

I recently added a Wrasse, no issues.

For me though, I'm full at 5 fish (although just one more Tang might be ok)...