This is 100% my concern without quarantine. From readings and research into old tanks, it seems like the 1 disease that will kill even the healthiest fish in a tank is marine velvet if it takes hold.I didn't QT at all for a while. It worked well for me...for a while. Several years ago I had a 180g almost completely stocked. I bought what I thought would be my final fish (3 flasher wrasses). Unfortunately, they came in with velvet, and I watched as over the course of 10 days, my fully stocked 180 dwindled to 2 fish that survived. Lesson learned. I hate quarantine, but I'll do it to avoid going through that ever again.