It's best to start with a healthy specimen for sure. But, once you get it down, you can be successful even with obvious illness. My powder brown and tusk came from a store where I observed multiple fish in the same row scratching and breathing fast. The powder brown I chose actually looked worse than the other, but ate where the other wouldn't.
I knew they had something, just wasn't sure what. I did a freshwater dip on both before adding to my QT and found flukes. Within 24 hours the tang was covered in velvet. I upped the copper (I use cupramine) over the course of about 24 hours to .3, and .5 within a day or two more. After a week at therapeutic copper, I started praziquantel for the flukes. I saved both fish, and they are both now happy in my DT: