I want to talk about quarantining, or not quarantining. I get in trouble for my opinion for this all the time (yes, even on this forum which is pretty liberal with your methods) and that is one reason I don't post on that big forum any more. People have the wrong idea and for some reason very few people grasp the concept. Quarantining if fine, if that is what you want to do, go for it as it is one way to run a tank. But if you don't want to quarantine, there are things you should do or you will lose your tank to disease. You can't just have a normal tank that you set up a year or two ago and decide to not quarantine. There is a process that goes along with it just like there is a process that goes along with quarantining and if you do it wrong, you will lose your fish and condemn the system you are using.
First and foremost your fish need to get into excellent shape, which we should strive for no matter what system we use. It takes about two months to get most fish into excellent shape if it is done correctly. I know it takes about 7 weeks with damsels so I am guessing on other fish.
Just because they are swimming around looking at you while whistling doesn't mean they are in great shape. If they are not spawning or making spawning gestures, they are not healthy no matter what their doctor says.
To do this we need to eliminate the dry, sterile foods like flakes, pellets and freeze dried anything. Use them in an emergency or if you go on vacation and you need to put it in a feeder but never as a way to feed fish every day. If you want to use dry foods, "keep quarantining". I mean that. ( yes, even expensive dry foods)
I wrote many articles about this so I won't go into the scientific research on it.
Next get some fresh frozen foods or the absolutely best, live blackworms. By fresh frozen I mean get something like clams that you can buy live (not if you live in Arizona). Buy the biggest live clams you can. Chowder clams here in New York can weigh a couple of pounds. Partially open the clam and insert something inside like a nail. This will make it easier to open the thing after it is frozen. It takes 2 days to freeze a big clam.
Open the clam and shave off paper thin slices with a knife. Make a dent like you would do in chocolate and slice off ribbons of clam. I supplement the clam with frozen Mysis, LRS food and live worms.
This diet, "every day" will get your fish into spawning condition. That won't necessarily make them immune especially if they have been in your tank for a long time or were quarantined for 72 days.
I think the best thing to do if you are really worried about disease is to keep your quarantine tank and put the new fish in there when you get them. Set up that tank with hiding places of rock, not pvc. Give them hiding places and light it as you would a regular fish tank. Feed them the diet I stated and observe the fish for a week or two but do not medicate with anything. If you see one or two spots on the fish, I also would not medicate as that is usually normal. Of course if the fish gets a bunch of spots, starts shaking and texting SOS there is something wrong and you would have to replace the rock with PVC and use copper.
I personally would use copper "and" a diatom filter as that would clean the fish of any parasites in a couple of days.
Remember, I never have to do this part because my fish don't get sick and yours probably won't either. I am talking about a healthy "looking" fish you buy from a LFS. That fish was probably in the ocean 2 weeks ago and has some sort of immunity in it's system. Our goal is to nurture that immunity, not suppress it, with the proper food and very low stress.
We want the fish immune, not sterile and to do this we need to get food into it along with "live" bacteria at every meal.
After a couple of months your fish should start to spawn (if you have fish that will spawn). Even fish that are not paired will start to clean a nest, bite you when you put your hand in there and chase other fish away.
If you do all this and if your fish are now in spawning condition, you can now add fish from an LFS without being concerned about parasites as now parasites should continue to boost the fishes immune system.
I have been running my tank like this for decades and I may have invented this system (along with the internet, unless that was Brian Williams) It works, I can prove it works and if you don't like the idea of this, keep quarantining as there is nothing wrong with that as long as you can quarantine everything and never want to use anything natural from the sea. Most inland people need to do that anyway.
Now you can write an article saying I am advocating Russian Roulette and don't know a fish from a duck billed platypus.