Over the last 5 plus years I have not lost a fish...well, just one. A Tusk jumped out and committed suicide as I left the cover off accidentally.
Besides supplementing feeding with vitamins, I also soak the food in garlic xtreme a couple times a week.
I have never used a QT tank...but when I first started in this hobby over 30 years ago I had numerous losses. maybe I have been lucky recently . I concentrate on vitamins, etc if a fish shows signs of disease. I really do not change my normal routine, other then soaking the food for a particular animal and making sure I do it more frequently. A few months ago I transferred some fish to a larger tank I set-up along with the live rock. My Niger Trigger showed signs of ich which baffled me since I had no addtions of any kind to my tank in about 4 years or so. Perhaps I screwed up the diagnosis but I know what it looks like and how fish act when they have the disease. In two or three days he stopped rubbing and the little white spots were gone and have since not returned. I suspect the move stressed him out which is why his immune system was weakened. I simply thought the disease was not present since I had no issues for many years...go figure.
I also try to keep my water parameters steady, with weekly 5% water changes and regular maintenace. I also try to feed all the animals the diest that is recommended. For eample, part of my tangs dies is seaweed sheets along with macro algaerom the fuge. I never feed frosen brine shrimp as it has little if any nutritional value.
The type of fish I keep I would need a very large QT tank in order for the fish to be stress free while in QT. I do not beleive it is benficial to a stressed out sick fish to be placed in cramp quarters without the comforts of home such as live rock and appropriate surroundings.
I guess most folks do not have a QT tank due to lack of space, lack of understanding regarding the benefits, or perhaps the Scretary of Defense refuses to allow another tank in the house.