Maybe it appears to be ich free because the occurrence of ich is so low as to be undetachable.
The problem (for sake of discussion-Please :) ) is that just because a deadly outbreak of any pathogen (ich or other) isn't happening does not mean that the disease causing agent is not present in the water. Assuming that TTM worked because you don't see ich (in full bloom) is an assumption.
The discussion by Paul assumes that the ich is present in all tanks but only shows up during times of stress. A carefully managed tank may never have enough 'stress' to cause the fishes immune systems to weaken so that the infection can get bad enough to be seen, or become a cause of death or sublethal concern.
I wore a mask all last year and had zero colds. I was not in QT, I was in a well managed system.
I got a vaccination last week and got crazy ill. The cure gave me the disease that I have already had before. I recognized the symptoms, it made me extra grouchy and argumentative. should I delete this post?
(I really like this thread. I hope my stupid comment doesn't ruin your day. ) HA!