i would recommend doing a water change every week to get it down if you have refuge get some cheato and place in your refuge, i would get a phos reactor (TLS) and get some de-nitrate media and you should be fine.
i believe your fish has red fin infection, i treat it before using melafix made by api, i purchase a yellow tang from ***** and within 24 hours it ws dead from that infection. you need to also buy a 10-20 gal tank for your qt.
just got rid of mine, i did a water change and got some nirate media, use the media bag, or a reactor, kill your light for 48 hours and use phosban also with reduce fish feeding