You cannot kill off ich in an aquarium in one lifecycle because not every single living ich in your aquarium is in the identical lifecycle stage.
That's true in a sense, or up to a point, and that's why you don't exactly see
instant ich infestation and coverage or
instant ich disappearance. So, no, it's not instant on, instant off, as though it's all one organism. On the other hand, I've had cases where time from first becoming visible to no longer visible on the fish was just a day or so (after which I agree you need to continue treatment for a while).
So the possibility of eliminating ich within a few days comes down to timing of treatment, type of treatment and temperature. Begin treating just before they start dropping off and continue long enough after they've all disappeared from view and you can potentially cover the two stages during which they're susceptible to meds within a few days or less and within what I would call their
collective life cycle. True, you might look at life cycle differently than that, but that's not my argument. My point is it's possible to kill ich off completely within a few days of actual treatment.
Look at the numbers--
reference article
I've had cases where all visible parasites disappeared over the course of 24 hours or less. After dropping off the fish the parasite spends just a few hours max before settling to the bottom, but while free swimming during this stage they're vulnerable:
The released non-feeding tomonts swim for 2 to 6 hours before settling on a substrate. (Nicholl and Ewing found that a light substrate was preferred to a dark one.) Some biologists count this brief interval as a fourth life stage (during which it is susceptible to medication, by the way, according to Dr. Peter Burgess, the resident "fish doctor" at Practical Fishkeeping magazine).
Reproductive stage in substrate may last from just hours to days (or longer), depending on temperature:
The tomont's time-span remains temperature-dependent: at common aquarium temperatures it's a matter of hours to days.
After this, the new parasites in search of a host survive a max of 55 hours at a cool
68 degrees, less at typical tank temps:
at 68°F none survived after 55 hours, according to the pioneer expert in fish diseases Wilhelm Schaperclaus.
Numbers vary for complete life cycle according to source. Various sources put it at 3 days at 80 degrees or thereabouts.
Fish channel
The duration of the life cycle depends on temperature, ranging from three days at 75 degrees Fahrenheit to more than a month at 50 degrees.
Fishlore
But once it's free-swimming, it can be killed. Since the life cycle of ich takes at least three days at 80 degrees to complete, ich must be treated for at least four days.
Probably why one product, Quick Cure, can claim to cure ich
in as little as 24 hours, no doubt a best case scenario. But get the timing right in the right conditions and: as little as two hours after dropping off the fish, during which the ich is vulnerable, it can settle to the bottom for reproductive stage. As little as a few hours later it can be free swimming and vulnerable again. Have the right meds in the tank at the right time...
Setting that aside, complete life cycle can 3 days, time from dropping off the fish (vulnerable) to being killed off while re-entering the water column (vulnerable) can obviously be less. Get the temperature right and timing right and it is quite possible to thoroughly eliminate ich in just a few days. Again, as I said already, I'm not saying you shouldn't treat more than a 3 day life cycle or
recommending to attempt to time it just right so you only have to treat for a couple of days. If just one survives and reproduces you potentially start all over again. All I said above is it's not impossible to eliminate ich in just a few days of treatment. I've done it before. Depends on timing, temperature, method of treatment, probably also the particular strain of ich,
