A LFS quarantine is not a real quarantine, and you stomping feet, or closing a door is BS, unless what you do makes them jump out of the tank.
And when you quarantine it is not just about what the new fish could have.
Any fish you have already could built a semi- immunity to certain things in your tank, that the new fish haven't resistance to. I quarantine any new fish in a separate tank for about 3 months before adding to a mixed community, and each day I add some water from the tank they will be placed in, so they get acclimated to the population of bacteria, and water parameters they will be exposed to, albeit slowly, and hopefully also build up whatever immunity is required. The 3 months also give me time to observe, and allow any bacteria or other malady to manifest itself. It can take time for bacteria to build up an infectious population, and appear to our naked eye.
But in the confines of a tank, all it takes is one infected individual to bring an epidemic in only a short time. I skipped time in a quarantine some years back, and this is what happened.
Since that time, I always do the entire 3 months, even though I wa able to successfully treat them. But ick is easy, some other bacterial diseases are not.