Whatever method you choose (TTM, copper, chloroquine, etc.), I cannot overstate, from personal experience, how important it is to quarantine. No fish gets anywhere near my display tank unless it has gone through therapeutic levels of copper for the requisite period (2 weeks if you move the fish to a second sterile QT tank, 4 weeks if you use one QT tank). I used to have constant disease issues and mysterious deaths. Then a velvet wipeout 3 years ago. Since I established my QT, I have had no issues in my display. In fact, it is a little boring now because no fish are dying and the tank is full, so I can't buy more fish!
Please note that QT itself can be tricky. You will lose fish quickly to ammonia and oxygen deprivation. Read up on how to make sure the QT is habitable. In my experience, ammonia is the killer in QT, not copper. I personally add biomedia from my DT, dose bacteria, AND do regular large water changes. That has cut down my QT deaths significantly.