There is a lot of information out there on Marine Ich, Including a sticky in this forum that would give you more information than I probably can. Ich is a parasite and had to be introduced some how to the tank by bringing in a fish with it. Wether a fish show signs of Ich or not it is always best to QT them because healthy fish may not show symptoms of the parasite. The life cycle of marine ich is described in many forums and articles which I suggest reading through to better understand this parasite. It is best to treat in a seperate tank with copper, hypo, or transfer method(The only proven methods). Also your main tank will have to be fallow(fishless) for 8-12 weeks to kill off any Ich that may be left in there. Otherwise all other additions are susceptible to this parasite. Did you purchase the clown fish without QT'ing them? What other fish do you have with the clown fish?