From my experience I have found that if you have a reef tank and only one fish is showing signs of having ich, it is better to just leave the fish alone. Whenever I have tried various "reef safe" remedies they have usually caused more problems than I had initially.
Also, if you have a reef tank then you probably have lots of corals and live rock. Well, i don't know if anyone has some good tricks to catch a fish but I can't do it without removing almost all of my live rock and corals just to catach the fish. Obviously this ends up causing much more stress on not only your already sick fish but could cause your other fish to get stressed and become sick.
So, in cases like this, I have found that the best solution is to leave the fish in the tank, raise the temperature to about 82 degrees, and begin doing daily water changes to lover the salinity to about 1.017.
I have needed to do this a couple of times and in each case the sick fish has recovered on his own. Every time I have go through the process of catching the fish, moving him to a QT tank, and medicating him has resulted in the loss of the fish and sometimes one of my other fish would get sick too from being stressed.