I think its impt to qt all fish even from fellow reefers. I learnt the hard way. Once I bought some clowns from a fellow reefer (they looked perfectly healthy btw), and put into main tank. 4 days later, all my banner fish got hit with ich.
Another time, i bought 6 fish from a lfs and put into my main tank. In the end 5 survived (1 KO due to stress) but........for the next 3-4 days, had to keep clearing corpses every morning.I lost about 15 of my long time residents due to bacteria infection. I think sometimes, the fish we bring back are carriers - they may have developed resistance to a particular strain of disease, but our own resident fish may be vulnerable.