If I had the space for a QT I would have one. Lets say I do have the room for a QT and the fish die. The LFS doesn't do anything about it anyway. It's like throwing money out the window. All my parameters are good. 0 across the board. Powder blue tang one year dead. Clown fish bought as pair, one died in a year. I think the other one killed it. Three Blue chromis added at the same time all died within a year. Six line wrasse lasted a month. Lawn mower blenny lasted three weeks. Royal gramma lasted two weeks in my son's nano tank. I would like to set up a QT but its not possible. I've thought of buying from live aquaria, but I'm not sure if I would be just throwing money away again. I know they have a 14 day guarantee, but after the time is up I would be out of luck again. Plus shipping costs.
I keep my QT in my master bedroom. It can be just a 10 or 20 gallon tank with a heater and hang-on filter.
How big is your DT, by the way?
As sandwi54 indicates, the fish that died within a month or so perhaps were the result of collection and handling practices at the source. They may have been caught with cyanide or left in buckets in the sun or poisoned with by ammonia if their water wasn't changed. These are difficult to avoid completely but you increase your odds by buying from a a good vendor such as Live Aquaria, Diver's Den, or a lfs that gets its fish from Quality Marine. These vendors attempt to avoid collectors with a poor track record.
The clownfish and chromis deaths could very well have been simply due to aggression. You typically are better with just one. People tend to advise that you will end up with one chromis no matter how many you start with, unless the tank is fairly large.
Looking at your list, only the powder blue death would seem to be the only one excepted from either bad collection practices or aggression. Your clown or the yellow tang may have hassled it or perhaps it wasn't getting enough food or particular nutrition (greens).
Let us know what type of clown you have and whether you have mobile inverts such as shrimp, crabs and snails.
I suggest you get a small quarantine tank, buy a hearty fish from Diver's Den, and manage water quality carefully with frequent water changes. Make sure the fish is eating well and looks fine for a couple of weeks before transferring it to your DT. Assuming that works well, try some from Live Aquaria. It will be critical to bring in compatible fish, so ask here for recommendations.
Finally, this hobby can be discouraging but hang in there, it is worth it. You will get it under control and end up with something you will enjoy.