As the others have said; when Cryptocaryon first gets started, all of the parasites are more or less "in sync" and tend to drop off at the same time. As the disease progresses, they increase exponentially, and become out of sync and you'll see spots all of the time. In some instances, in tanks with very good conditions, the Cryptocaryon drops off all at once, and none of the tomites seem to make it back to the fish and the infection goes into remission. This is also the reason behind many reports of "reef safe" medications working. Don't count on that though! Observational quarantine is something I only use for specialty fish - flashlight fish, garden eels, etc. Here is an excerpt from my upcoming book on that topic:
“Observational quarantine” is a term you might hear. It involves a technique that is used when one wishes not to expose a new specimen to any procedure more drastic than environmental control and observation pending the development of any obvious disease symptoms. The use of this technique should be limited to extremely delicate fish, such as
flashlight fish, or fish that do not normally transfer diseases to bony fishes, such as sharks and rays.
The specimen is placed into an isolation tank and observed for signs of disease for a period of 45 days. The difference between this technique and no quarantine at all is that the problem of disease transfer is minimized on a case-by-case basis in choosing "low-risk" or "low-value" tankmates. In addition, the operative word observational is very important. At least twice daily, the animal must be inspected closely for signs of developing problems. Please do not fall into the habit of routinely using this technique simply because it is easier—because it is certainly not very effective.
Jay