There are tons of threads on this forum about ICK and many suggestions on how to prevent and treat it.
One preventative method I use which several do not agree with is freshwater dips before introducing them into the tank. Bring a bag of freshwater up to tank temperature with a bit of chlorine remove in it. Float your new fish in the tank for 20 minutes or until he is equal to tank temp. Add a little water from the tank into the bag if you wish. Then transfer the fish to the freshwater bag for 2 to 2.5 minutes. My LFS
guy recommended 5 minutes, but it depends on the fish. Watch him for extreme signs of stress. Then transfer your fish to the main tank.
I have used this method and have never experienced ICK. Like I said, some people will disagree stating this is to stressful on fish and that you should use a QT tank for three weeks and monitor. I would rather have a fish stressed out for 2 minutes than have to go through the medicines for weeks and possible worst, die on me. Your call.
P/S The rant above was geared for preventive measures. I would suggest making sure you have completely eradicated the ICK from your tank. ICK will stay in your tank as long as there are fish present. You need to move all your fish to a QT and just let your tank sit empty for about 4 or 5 weeks until the ICK has gone through its cycle and dies. Good luck.