Hospital tanks aren't necessarily the first and only response. Having gone thorough this situation quite a bit over the years, all options are risky - and each has advantages and disadvantages. In my experience, and this is just my specific circumstances, I've found it best to just leave the fish where they are, making sure they're comfortable/stress-free and well fed with healthy diet items. The only change I make when ich is present, is to feed them medicated ich food and sometimes dose reef-safe Ich Attack medication, depending on how bad it is. I'm still not sold on the medicated food or ich meds for their skin. I do have confidence in the feeding though - feeding healthy foods that are the particular fish species' food staples, like various types of dried seaweed along with frozen and quality dry foods for Tangs a few times a day and ensure the tank has all the creature comforts such as caves, crevices, hiding spots, good water quality, etc.
I've found more often than not, that going this route provided the best chances for survival. The ich might persist for weeks or months (on and off) depending on the severity of the initial infection and number of fish in the tank. But, over time if the fish are overall happy, the ich cycle reaches a point where all the fish have developed enough immunity and lack of stress to break the cycle so that the ich can't find a single host to maintain their numbers and then die out. I've had reef tanks where ich was present, but then never came back - years later.
Using hospital tanks are a pain. You have to stress out your fish with the retrieval from the main display, possibly damaging/destroying other things in the process. Once you retrieve the fish, then you have the issue of keeping your hospital tank's parameters stable - assuming it's not a permanently established aquarium, etc. The only time I've seen hospital tanks work consistently effective is when it's well established and permanently setup for just that purpose.
