Doing a freshwater dip with the fish would be the safest way to cure ich if you have coral in the tank. If you add copper to your tank your going to risk loosing corals. If I have a fish with ich, I simply just get a bucket of RO
/distilled, make sure the temp and ph match the tank(VERY important), put the sick fish in and let sit for 30 mins. Be sure you watch the fish closely) tangs always seem to play dead but they are not harmed in any way. A gentle touch will prove it. This might seem cruel but it doesn't hurt your fish and the cure takes care of all kinds of problems. That usually does the trick.