Sorry to hear of the troubles. First, you should be clear that the UV does not prevent nor eliminate diseases. I mentioned its value to control bacterial blooms and large populations of water-borne micro organisms in previous posts. The value of UV are for water clarity, reducing the number of micro algae and bacteria in the water, and the disruption of proteins.
If you have the correct diagnosis and the fish are infected with Marine Ich (Cryptocaryon irritans), then there are only three known, for sure, treatments. They are copper, hyposalinity, and the transfer method.
The display tank cannot be treated by any of these methods. So the way forward is to remove all fish from the display tank and put them into hospital tanks. The parasite, fortunately, dies off without a fish host. The display tank goes fishless for no less than 8 weeks and then is Marine Ich free.
In hospital tanks, you treat the fish with one of the above three methods. Once the fish are clear of any visible signs of the disease, you continue to hold them in the hospital tanks for another 4 weeks for observation and verification. After both the display and the fish are 'cured' then put the fish back into the display.
We know more about Marine Ich (Cryptocaryon irritans) than about any other marine disease. Not because of the hobby but because of the millions spent on it from the aquaculture and fish farming industries.
This is only some of what we know about Marine Ich (MI), whether aquarists choose to believe it or not:
1. MI is an obligate parasite. It can only live and reproduce with a fish host.
2. Some fish have defenses against the parasite that gives them the opportunity to develop an immunity of sorts before the fish dies from the parasite. Consider this -- if the parasite kills all its hosts, the parasite will die too.
3. In the aquarium, the fish can't get away from the parasite so the captive nature of the aquarium benefits the parasite by shifting the 'natural' ratio towards many more parasites than fishes.
4. Regarding the reverse of 3. -- In the wild the fish travels around and can get away from large numbers of the parasite.
5. Any bony marine fish, marine water, rock, substrate, etc. from the ocean can contain the parasite in one or more of its life stages.
6. Regarding 3. -- Even a fish with great defenses cannot always handle a large outbreak of the disease.
7. Regarding 2. -- A fish in captivity can lose its immunity and defenses built up to the parasite such that a few months later when again exposed to the parasite, can die from an attack.
8. A fish can have the disease and the aquarist won't know it. The parasite primarily targets the gills where more water passes through than over the body parts. The aquarist can't see an infection in the gills.
9. Regarding 8. -- The only stage the parasite is seen is when it is at the Trophont stage (visible at the end of this infected stage as a white spot on the fish just before it drops off) on the skin or fin of the infected fish.
10. Thus regarding 9. -- A fish can even be infected on its skin and fins without being seen by the aquarist.
11. Despite all these characteristics, the parasite can be prevented from entering into any display tank.
The observations you've made and experience can all fit comfortably into what we know. However, there can be disease-free aquariums. My home aquariums have been MI free since 1970. More than 1,000 fishes later, still MI-free. I quarantine everything, just like is done at all public and private aquariums.
As long as there is MI lurking about in the display aquarium, there is always an opportunity for it to bloom. You are right in one sense about the stress. A stressed fish is more likely to die from the disease. It simply runs out of energy reserve before it can develop immunity or before it can survive the onslaught. But remember MI is an obligate parasite. What most people mix up is the ability of the fish to get infected with the fish dieing from the infection (where health and condition come into play).
Every fish whether healthy, stressed, sick, or strong is equally able to get infected. The weak ones just die quicker or the stronger ones IF they have the physiology may survive the attack. When it comes to infection, MI doesn't care if the fish is healthy, stressed, or sick.
Think of MI more along the terms of a Tapeworm. If a human gets the Tapeworm eggs inside, it doesn't matter if the human is healthy, sick, weak or strong. The Tapeworm takes up residence in the intestinal track. Does a sick person get them inside easier than a healthy person. No. Each kind of person gets infected by swallowing the eggs or eggs going through the skin, regardless of their health. MI parasites infect the weak as well as the strong. What is observed is that the weak die quicker or those in between show the symptoms.
The quarantine process should be less stressful than the fish being in the main display tank. What humans tend to do is project their emotions onto the fish. The fish 'looks unhappy' in the QT. Wish I had a dime for. . .
If the reader can get into the quarantine process and provide the needed optimal conditions, the reader will find that marine fish being alone in a properly sized and controlled high quality environment, being hand-fed the greatest and most nutritious diet, is the greatest thing that can happen to a newly acquired wild caught fish since it was 'yanked' out of its home.
Now let's talk about the fish that 'recovered.' What did that fish go through. Humans forget that fish feel. More studies are being done to verify the initial work which indicates that fish can 'feel.' (This will turn the whole fishing sport in turmoil--maybe! ) A fish with MI in the beginning stages is a like a human with 2 dozen mosquito bites. How does that feel? Only these mosquito bites don't all go away. They keep getting replaced until the body finally starts getting ahead of the reappearance. I don't and won't put my fish through that. I liken the 'let's wait and see approach' to "Let's see if grandma recovers from pneumonia and not take her to the hospital."
MI infection is very easily prevented and kept out of the home aquarium. Most LFSs just put copper or chemicals in their system so that the fish looks good enough to get it out of their tanks and into the aquarists' tanks. The fact that many Marine fishes come to them infected is of no surprise since the fish are kept together with other fishes along the way. So they will run the copper in their system (which the aquarist can test for, BTW) and deny it to customers or only tell those who pin them down. But the MI parasite does not have to be in the display tank. That is up to the aquarist. It is rumored that it can't be stopped by mostly those who don't want to fight it or prevent it. It is especially claimed that MI can't be stopped by those who run a bad quarantine process and blame it on the parasite rather than on the person running the quarantine
