That's easy. That's an 'old question.' Marine Ich (
Cryptocaryon irritans) is only present if you let it into your tank. That's all there is to it.
The "life cycle" set at 4 weeks is a problem. Science and Nature likes ranges. Marine Ich (MI) has high and lows. The 4 weeks is only an average. As you increase time, you decrease the likelihood of the disease being around. If you choose 8 weeks, then you have about a 99.99% chance that all the MI is dead.
BUZZZZ! Wrong answer!
Seeing is not believing. Here we go again.
MI can reside in the gills -- out of sight of the aquarist. In fact the gill is the most likely place to find MI. Why? Because fish 'breath' by passing water past the gills and when that water contains the infectious MI phase, then there is a greater likelihood that the MI will get to the gills before it gets to the body/fins.
NEXT: What you see as a white spot is actually only the
end of the MI phase. This is when the MI is 'pregnant' and ready to drop off. When the MI is just burrowing into your fish, and before it becomes engorged, you can't' see it at all with the naked eye.
The above are the reasons why I changed your text to actually know whether or not the fish is diseased NOT whether or not you can see it.
Regarding: I also say "disease free aquarium" because if the fish have been visibly disease free for a year, then none of the fish dependent parasites could survive without making their mark on the fish. You are missing another fact.
When no new MI is introduced into a tank, the MI already there go through multiple generations. It turns out that after about 10 months, (15 or so generations) that the MI is too weak to infect fish. So, your "year" would be beyond the ability of the old MI to do any damage.
Your last paragraph doesn't fit what I meant to say. Maybe it is not clear. I'll take a slap for this one!
I was not meaning the QT had to have UV. UV is a control, not a killer or cure for MI. MI can live and re-produce in your display and still be kept under control from large blooms of infecting free-swimming MI by the use of strong currents, UV, ozone, and other 'things' which inhibit or 'control' the MI from becoming a massive outbreak.
Any better now? :rollface: