I am with you, I had been quarantining and treating, but somehow still had ich show up in my tank. I tried to catch all my fish. Those that I caught and put into my hospital tank ended up dying with the exception of my Purple Tang. My hospital tank was in good shape from a parameters perspective. I believe it was the stress of capturing and moving the fish that did them in (basically failure to thrive situation after moving). I never could capture my Blue Tang from my DT. So I did a lot of research on ich management. That all happened 8 months ago. When I saw that my PT was stressed, I moved him back to the DT and left things alone, he was covered in ich. I had purchased a couple pounds of black worms and started cultures of white worms. I also invested in a UV sterilizer and re-plumbed my system to ensure the that UV was at sterilization levels. I took a two prong approach, focus on the core health of my fish (feeding the worms made an incredible difference in color and even behavior) and then managing the levels of ich in my system. I introduced the UV sterilizer and then also treated the fish with Seachem Focus and Metroplex for approximately 3 weeks. The symptoms of ich did reappear during the 30 day period, but the second time around where I could see the dots they were significantly less. What should have been the next cycle I saw no other symptoms on any of my fish, including my BT and my PT. I run the UV 24/7/365. I have since introduced new fish; however rather than medicating the new fish I simply get the fish eating the worms in quarantine. Once they are eating I introduce them to the DT. I haven't had any of those fish become symptomatic with ich which I believe is still in my system. This method seems to be working for me, I am not saying it is a sure thing; however, I think there is something to be said about focusing on the fish health and ich management. To close this out I have had a couple of "stressor" events... power outages for extended lengths of time 12 hours and 8 hours. Temps fell in my tank, I have battery backup for my pumps (I also now have a generator). The fish never showed signs of symptoms and are thriving.
As a proponent of natural filtration producing in tank live food to promote healthy immune systems in both fish & coral. I have not quarantined in 35 years of 47 years in keeping marine tanks. I have seen ich twice in my systems. The first time I saw ich was on an established mature 10 year old system that had not seen any additions in 2 years. Following a loss of power stress event, I saw Ich everywhere in 1000G extended system. I reestablished stability and ich disappeared. Ten years later, I saw ich thru shipping bags on seven tiny Hippo tangs. Fish were sideways and showed great stress in color loss & eratic breathing. I made an executive decision and immediately released tangs into fife teen year old tank that had ich five years earlier. Scratching & itching behavior was evident on all new tangs, four showed obvious ich spots in the shipping bag after a 90 day quarantine at DD. After 3 days, all ich spots disappeared. After 7 days all scratching & itching ceased. That was ten years ago. Do I have dormat ich in my 25 year old tank? Probably.
After I went through the first ich episode 25 years ago, I started reading scientific papers on ich with the wisdom at the time for fallow period was 14 days. The days to fallow have increased from 14 to present
76 day fallow period. The fallow period has been increased because it is not absolute. I continued reading research paper in detail and one phrase stuck out, “most cyst will hatch in X days”. Nature will survive.