I've looked very deeply into the “ick issue" because my FOWLR has has repeated deadly outbreaks that I'm determined to put an end to. I'm not an expert on this...that's for sure...but I think that a lot of confusion about it stems from the fact that it may or may not be coupled with another disease...Oodinium. Oodinium is sometimes also referred to as "ick" though this might be an incorrect reference because the white spots that can appear along with it might instead be due to a secondary "ick" (Cryptocaryon) infection. Oodinium is also called "marine velvet" because of the kind of coating an infected fish will get on its body as a result of this deadly disease...an Oodinium infection will, very often, also cause an infected fish's stamina to fail so that it also gets victimized by the "ick (Cryptocaryon irritans, that's referred to in the article linked to above.) if that's in the tank, even if no fish had yet been infected by it before Oodinium appeared. Hence, the appearance of the white spots. But, it should be noted that an Oodinium infection of a fish may very well kill it without the appearance of any white spots at all.
IMO, from having had appearances of ick (Cryptocaryon) on a few occasions in my reef tank, which were always left untreated and the fish always recovered safely on their own, ick is not nearly as deadly as Oodinium is. And, it is possible that a reef, which is well-stocked with coral may have some kind of capacity or characteristic that lessens the vigor of a deadly ick (Cryptocaryon) outbreak.
But, if Oodinium appears in a fish tank, and there has been ick (Cryptocaryon) in the tank even if no fish had been infected, then it is almost certain that if the fish are not immediately treated with copper in a quarantine tank, the Oodinium will kill them or the ick will have time to appear and the the fish will succumb. It is that virulent.
So...from my experience...and it is significant given the repeated bouts I've had with both Oodinium and Cryptocaryon in my FOWLR (as opposed to my reef)…the most provident approach is to quarantine every newly acquired fish. Ideally, as stated in the article thta Kathy posted above,) for a period of six weeks.
Now, I know very well that this is a lot to ask a reefer when a new specimen is obtained. But, that period of time in a very mild copper solution in accordance with the manufacturer's directions, will assure that any infected fish will be cured of any possible Oodinium infection that hasn’t yet manifested itself, as well as any ick (Cryptocaryon) on the fish and so it will not ultimately be introduced into the display tank.
In the alternative, the fish can be placed in untreated ("copperless") water and observed for a period of three weeks and if no symptoms appear, it is in all likelihood uninfected and can be placed into the display tank...though, there is some risk that the fish has is carrying ick (Cryptocaryon) and will carry that pathogen into the display tank with it.
Placing the newly obtained fish into the "copperless" quarantine tank, on the other hand, will quickly determine if it has been exposed to Oodinium because it is, unlike ick (Cryptocaryon,) IMO, almost completely assured that the disease will effect the fish. (Some fish are not effected by Oodinium but will carry it assuring futute infections if that fish is not treated with copper.) If the “velvet coating” appears, the fish begins to “cower in a corner,” and the fish fails to eat when in the QT, it almost assuredly has Oodinium and copper treatment should immediately ensue or the fish will die. If none of those symptoms appears, then the quarantined fish should be observed for ick (Cryptocaryon.)
Because of the deadly jeopardy that a fish can be placed in along with all other fish in the same tank if Oodinium is present...with or without ick (Cryptocaryon)...I've concluded that the best practice is to place all newly obtained fish should be quarantined in a copper treated water solution as described.