OK well,,,,,,Not to belittle some posters. But a FW dip will kill parasites and cleaner shrimp also help. I do not suggest you do a FW dip because in my opinion it is to stressful on the fish. If indeed you know your fish has ich and you want to treat the proper way. I would remove the fish from the main display and place it in a FULLY CYCLED quarentine tank. I would then treat the fish with Hyposalinity NOT COPPER. In my opinion copper is also to stressful. You will want to leave your main tank fishless for about 8 weeks and leave the fish in Hypo at about 1.009 for the same time frame. The trouble with this method is maintaing a proper PH so keep an eye on all your parameters if you choose Hypo. Now all that being said. I would not treat just because you see scratching. It is a sign but not a definate. I have had an Atlantic Blue tang for 4 years that every so often will scratch. He has no signs of ich. Also keep in mind you are going to get several opinions on this site. So take what you hear and make your best decision. Good luck. BILL