All of this.
There are so many issues tangled together here. I would also second what MnFish1 just posted - that the "method" Paul recommends is sound. It is the method I basically follow. But the conclusions Paul arrives at don't always seem that sound to me.
1. The bacterial make-up of the digestive system is incredibly important. And diet affects that bacterial make-up. Your stomach will have "better" bacteria if you eat fruit and vegetables rather than skittles. Killing that bacteria with prophylactic dosing *may* do more harm than good. Before my mother passed she was repeatedly treated with antibiotics for several infections. Each treatment wiped out her digestive bacteria - which created a number of other life-threatening issues. But it happened to be the case for her that it was the lesser of two evils. The decision to treat a fish with antibiotics will depend on how serious you believe the infection or illness to be. That said - people and fish can replenish their digestive bacteria. Probiotics are helpful in this regard. It is not the case that gut bacteria will always be compromised after antibiotic treatments. It is also not the case that consuming parasites or diseased material will help ward off parasites or diseases. Which is why people don't eat tapeworms and cancerous tumors. A good diet is all you need. And all your fish will need.
2. Exposure to diseases is generally not a good thing for individual people or fish. It may be helpful for a population over several generations. But it is rather a mixed bag for an individual. You might develop some short-term immunities. But you might also just get sick and die. The example of doctors and nurses developing immunities is tossed around a lot. Sure that happens - but I've never seen any evidence that doctors live longer lives than anyone else. No one, doctors included, are eager to be exposed to infectious diseases. Hence the masks, gloves and surgical gowns. There is no sound reason to deliberately expose an individual fish to parasites and disease.
3. Paul's tanks appear to have always been more "diverse" than most people's tanks. By that I mean he is always adding microfauna to his tanks - particularly mud from the shore. He notes that he only recently noticed sponges in his tanks. But my tank is nearly as old as Paul's and my tanks have *always* been filled with small tube worms, sponge-like organisms, tunicates, tiny barnacles and many other unknown filter feeders. You can't help but acquire these organisms if you are adding live rock, mud or sand to your tank. And these filter feeders consume free floating organisms. The more densely populated a tank is with filter feeders, the less likely parasitic organisms can complete their life-cycle. It is also worth noting that reef tanks have several flow "choke-points" where all the water is forced through one zone and the filter feeders grow in clumps. A condition that does not necessarily happen in the wild. And these choke-points are killing zones for free-floating parasites. Reducing your fish load will also reduce the chances of parasites finding a host. Without exception, all the people who don't seem to have trouble with ich and other parasites seem to have tanks with a large proportion of filter feeding organisms relative to fish. The flip side is that public aquariums and reefers with a large number of fish and a low to non-existent number of filter-feeders do seem to have issues. Because adding a parasitic organism to a tank with a lot of fish and no predators is not going to end well. The sane approach is then to quarantine. Paul's fish are healthy because he feeds them well and because his tank is actively filtering parasites out of the water. It is hard to beat the effectiveness of such a biological filter. But honestly - 11 to 12 years is not that long of a life for a captive tang. My yellow tang has been with me for over 25 years. Other "methods" are just as effective as what Paul is espousing.
The need to quarantine will depend on your set-up. Fish are not immune to all diseases. Treatments can help. Treatments can also cause harm. Feeding your fish good food helps. Feeding your fish parasites is not helpful. Deliberately exposing your fish to parasites and disease is unlikely to be helpful. Adding clams, tube worms, sponges, tunicates and other filter feeders to your tank is a great hedge against parasites. Find the method that works best for you.
