I would echo what many people in the hobby will say about treating in the display. DO NOT DO IT. There are tons of treatments that are reputed to be completely reef safe. The problem is, everyone I have come across also has at least a few stories to the contrary. I would also caution about two additional points. First, your display is teaming with beneficial and non bacteria of all kinds (gram-negative, and positive). Treating the entire tank with something that will kill what is on your fish will also kill good things. We don't want to see a tank crash just for a single fish! Second, if you don't have a QT already set up and cycled, putting your fish in a completely new tank will cause some additional stress. I bring this up because I'm in the middle of the exact same situation. I didn't even HAVE a QT! It has taken lots of time/energy/ and money to keep my little coral beauty alive. For some time after getting her into QT with a damaged pectoral fin and secondary bacterial/fungal issues, I had to do water testing and changes daily. Testing was at least twice a day. The natural nitrogen cycle includes some considerable ammonia spikes. Not good for the fish. I just want to make sure you prepare yourself. If you are able to get some water today, set up the tank and let it sit over night. Never put a fish, especially sick one into a completely fresh tank. Also, one of the things I learned the hard way is to have a tank medical kit already in hand. I have gone to the store and ordered more things online in the past month than care to admit. Not to say I used all the things I got, don't want to over medicate, but from day to day, you might rethink your diagnosis. All the best for you and your little one. Hope it goes well.