I was plaged by either velvet or ich for the first 5 months of starting my tank up... I bought two clown fish that had it and they died from it (didn't use to QT because I never did with freshwater and was lucky for years). Bought a Kole Tang and shrimp goby pair, added them to the display a couple weeks later, the tang got sick and died.
I then set up a QT tank and spent a full week trying to get the Goby out to treat and QT, I finally got him and was so relieved. Treated him and let the tank fallow for a full 90 days. Added him back and started QTing some new clowns and added them once they were finished and all has been well since.
Now I QT AND treat every fish before adding it to the tank. I also QT all coral and Inverts before adding unless I get them from someone I know and trust. Learned my lesson the hard way as most do I suppose. Definitely wasn't a fun one, nor was it a nice introduction to the hobby. It did give me plenty of time to focus on coral and I learned a lot during that time so all's well that ends well!