Well let me say you are not alone when it comes not just putting a fish in your DT without putting it into a quarantine tank first. I believe if most people were honest, you would be surprised at how many people don't actually have a quarantine tank. I was one of those, I too put a bunch of fish in my new tank without QT'ing them first, everything looked great for a couple of days, fish were eating and swimming around in their new home and seemed to be loving it. Then one afternoon about 3 days after i put them in it was like someone took all my fish out of my tank and sprinkled salt on every one of them as some sort of practical joke. I started out with a powder brown tang, coral beauty angel, yellow tang, orange spotted goby, and an ocellaris clown.
Out of the original 5 fish i had I now have 2 - neither the coral beauty and the yellow tang are showing signs of ick, in fact the last sign of ick was about a month ago. So i figure the dreaded ick is gone right??? Recently I purchased an anthias and QT'd it for 3 weeks. All was well, decided to put him in DT. For 1 whole day he was fine, swimming and eating good, seemed to be getting along well with the other 2 and on the second day...BAM, he is covered in ick.
What am i doing about it? I have decided to treat ick the proper way and treat my DT the proper way. If I don't it doesn't matter what I do as far as a quarantine tank. If my DT has ick then whatever i put in there will have ick. I asked a known "expert"

ichthyoid what i should do and he so kindly replied with a very detailed description of what I should do step by step. I have decided to listen and do exactly as he said. Here is what he wrote, I suggest you do the same or else every fish you put in your tank will get ick and it will be an uphill battle from the moment they enter your tank. Sure they may fight thru it but they also may not, I am no longer willing to take that risk.
Ichthyoid wrote,
First, the term 'ick' is often used to describe many different diseases. These are most all parasites that live on fishes. Often it is actually cryptocaryon or brooklynella on salt water fish.
If you remove all fish from the main display tank for about a month, you interrupt their life cycle and they die. I would go ~6 weeks to be sure. You don't want to do all of this work again.
During this time, I would treat all fish with quinine sulfate to eliminate the parasites totally. These parasites can exist as a low grade infection on existing fish if not treated, and can devastate any newly introduced fish.
After this, you should not see any further 'ick', as long as you treat all new fish before they go into the main display tank.
These steps will require a separate 'hospital tank' for treatment. For this I use a bare bottom, with some large pieces of PVC (short pieces of pipe will do, or the fittings). This will provide structure for the fish to hide in and reduce stress during treatment. The pvc is also inert. Low light is good also.
Quinine sulfate is available from National Fish Pharmaceuticals. You can google them.
So,
1) put fish from display into hospital tank
2) eliminate parasites in display by waiting a month+ without any fish
3) treat all existing fish in a separate 'hospital tank'
4) after the month+ reintroduce your now healthy fish
5) treat ALL new fish before placing in the main display
6) do not allow any tank water from another aquarium into your display
7) use fresh salt water to 'rinse off' inverts, corals, etc., prior to placing in the display. You can use a rinse tank or pour the water over them. This helps keep new parasites out of the main display.
Be careful treating fish in a hospital tank by doing partial water changes. The hospital tank will have no bacteria to cycle nutrients produced by the fish, so the water changes will help keep water quality good. Otherwise, ammonia build up can kill them quickly. If the fish start breathing heavily (gasping at the surface is worst case) this is usually why.
Hope this helps.
I hope Ichthyoid doesn't mind me posting this because every new guy or gal just like myself needs to hear this and take the appropriate steps. Believe me this sounds like my worst nightmare because I have to move coral, take 90lbs rock out of my DT (maybe) to simply catch 3 fish. I am going to have water everywhere, have to put my rocks back in my DT which no doubt will not look the same. But I am going to do it because I can't live with ick on every fish i put in my tank. Sure you can do focus and metro but this will not remove your original problem, only help your fish to make it thru and maybe live with ick...not for me no thks, i am done with it and have decided to do what i need to do to get rid of it.
Hope this helps.
Dale