I live with it in my tank and it hasn't been an issue for me in several years. That said, I avoid fish like Hippos and powders that are known to be very sensitive to it.
FWIW, you're not the only one I know that has experienced this same scenario. I will just leave it at that.
Same here, though I avoid all tangs - period!
Yours isn't the only "did everything right and still got ich" tank. I think it happens a lot more than we like to think. You probably won't ever know how it got in, and I'm pretty sure that a lot of people who claim that their tank is ich-free are full of it. Not all, but a lot. People with strict qt also tend not to feed cheap food, and stock mindfully, so who knows? It's total speculation.
My qt is way more about catching brook / velvet / uronema etc. than ich. But I don't see any reason not to do easy stuff like ttm, if you can keep it low stress. I think a lot of fish die from qt stress and we just chalk it up to them being weak and are happy they didn't infect the dt when actually they would've been fine if they were put straight in.
I really don't think there's any right answer, except that all those "ich blaster exxxxtreme" potions at the lfs are scams.
+1
the 72 days fallow will kill it is a rather unsubstantiated myth that is just based on one scientific observation which by no means precludes longer times. To my knowledge there has been no exhaustive testing that would justify such claims, yet it is preached like the word of God.
I'm pretty sure there are ich strains that can hang around for much longer than 72 days - I would wager a bet that there are some that can lay in wait for a year until next years monsoon season stirs up the mud again and weakens the fish enough to be susceptible for a new rounds of infections.
I also don't think this is all by statistics or chance. I rather think that an ich cyst might actually be able to detect if there's a fish in its vicinity and only pop then. Others may react to changes in temperature or dissolved oxygen as it may indicate a storm. Should this be true, it may make all fallow procedures pointless.
IMO, if you absolutely want an ich free tank you need to sterilize it reliably and then only add fish that have been through TTM at least twice. Any coral with exposed skeleton or hard base may bring it into your tank. Same goes for shrimp and crabs, thought they should be clean after a molding.
Aerosolized water droplets containing ich has been shown (in the scientific literature) to enable infection of adjacent tanks. I recall it was something like 1-2 feet these droplets can traverse... However, ich cannot live without a host, so the idea that ich is floating around everywhere in the air is nonsense (unless there is an infected tank nearby).
The aerosol transmission, while certainly possible, would likely require a heavily infested tank with a lot of very sick fish. Otherwise the odds for a parasite making it into a droplet and then to another tank are rather small.
The reports of these occurrences came either from an ich research lab or a fish farm where I could imagine this to be a realistic problem. In a home setting, unless the tanks are right next to each other and use airstones for circulation, I see it as a rather remote possibility, especially if there is no big outbreak in one of the tanks.
As for the original question:
My system has ich in it for sure and I rather immunize my fish so they can handle it than have totally clean fish that may get overwhelmed easily should ich one day make it into the tank