This is a problem that happens to a lot of people. First thing you're gonna do is get them out of the main tank and place them into a QT tank. The next step depends on the particular fish. The best method for killing ich is copper. However, some particular fish can't handle copper (ie tangs, some angels, etc). For these fish, I would raise the QT tank temperature to around 84 and drop the salinity to about 1.016. The fish will be strong enough to handle the temperary condition whereas the parasite will not. When dropping salinity and raising temp, dont drop more than .02 in SG per day and dont raise more than 2 degrees in temp per day. Leave the fish in QT for at least 3 weeks. After QT, I like to give my fish a freshwater/formalin dip before entering the main tank. Good luck