I had fish for years, and I never remember any LFS recommending adding a chemical to an established tank for new fish. A lesson learned is to buy fish from a reputable LFS that quarantines and pre-treats their new fish before they are sold. With that being said, I agree with the posters that the best protection is to have a QT setup so you can do prescreening for disease before moving them to a beloved tank. While in the QT, if you see a disease, you can treat it there. Quarantine for 2-3 weeks to be sure.