I am sure every situation is different. My fish were fat and healthy. 2 that died I had since I started the hobby a year and a half ago. I have a 220 gallon tank with 6 fish in it. No over crowding, no fighting everyone doing great. I run two big uv’s and a ton of automation with daily water changes. I feed way better than most folks, twice a day and a mix of all frozen, brine shrimp, mysis shrimp, rods food, frozen copepods,squid and various frozen veggie foods for tangs. Everything mixed in jar with fish amino acids, garlic and selcon. nori strips daily. Powder brown showed first spot and yellow eyed kole tang shortly after. Yellow tang was last to show anything. Everything I read was as long as they are eating good they will be fine. They ate very well. kole tang would get more and more every so many days until it finally overwhelmed her. Bicolor went down very quickly in just a few days of showing it. Powder and clowns were next looking like death warmed over. I have no doubt if I left them all in to fend for themselves they would all have died. I regretted every post I ever read of all the people saying to leave them be and they will build immunity. I call bs all the way, I think some get lucky and are just a ticking time bomb when first power outage happens or some other event. Only survivors are yellow tang and powder brown and that was only because jay took the time to coach me through treatment. I am sure older tanks may have something magical that helps them do better (at least that’s what I keep reading) but my 4 month tank was no match for them.
As far as fallow goes, this my very first time doing it so I have no idea if it will be successful or not. I can only hope since alternative for me will be tank tear down and reboot.