QT is an absolute must in this hobby. Especially when your fish stock in one tank costs $500.00 or more. Some people, including myself take it much more seriously than a hobby, and for some it's their occupation. I've haven't seen one LFS ever that hasn't had at least one sick, dying, or dead fish when I arrived.
My fish setup starts off with a 20L which fits most fishes needs for a couple of months. For a biofilter, I use a 2 RODI canisters filled with bio balls or other high surface area plastic media. I can add/remove activated carbon as needed. Basic lighting and heater. If the tank has been offline for a while, I add ammonium hydroxide to water to make a 10% ammonia and water solution to add to the tank to jack the bacterial load up for fish. Basic household ammonia will work fine as long as its free of soaps, scents, and color. You would be surprised how toxic a couple of drops of ammonia can be in a 20L tank. I put a foam intake filter on the biofilter. The foam filter has been in my sump for a while to provide the starter culture.
I also add a couple of ceramic decorations that won't absorb any chemicals you add to the system. They provide hiding spots for your fish.
As someone said earlier, I use cupramine prophoactically to make sure ich doesn't make it to any displays. If you have copper sensitive fish, use only 3/4 of the full dose, and take twice the time to raise the level to 3/4 strength. I run lower strengths for longer as well, 3 weeks if everyone is doing well. If you have to do an emergency water change. Just add the proper amount of cupramine to the make up water and then do your water change. Test kits are good tools, use them.
After the cupramine, I do a 50% water change and add activated carbon to my BRS dual canisters, and leave that for 3 days. Remove the carbon, then I hit them with prazipro, for the recommended duration, or a little long depending on the need. I do another water change, and carbon, they rest again for a few days and then the head to the display.
I typically feed frozen enriched foods to keep their appetites up, and no treatment happens until they are eating. Unless special circumstances apply. I always remove the excess food with a brine shrimp net.
This setup has worked for me for years, and I can honestly say, I haven't had an ich outbreak in any display tanks since 1997. It is tedious, but very successful. Some fish require different approaches, and the setup is for the majority of fish the can handle at least 3/4 doses of cupramine.
Some people say that prophoactic treatments aren't necessary, and possibly harmful. My argument to that statement is simple. In the united states, it is against the law to skip prophoactic treatments for our pets for communicable and deadly diseases. I see cryptocaryon irritans as it is, a disease that spreads to our other pets and quite possibly will kill them. I see no differences between canine rabies, distemper, parvo, or crono viruses as examples. Not all pet diseases are communicable to humans, but for the safety of our pet populations at large, they are treated. Fish are the same way. They get treated for the well being of the population as a whole. If our fish could climb out of our systems and get the neighborhood fish sick or dead on their own, you can bet your salary that laws would be past to destroy these disease before they have a chance to do damage.
Hope this helps.