Poor or lower than optimal water quality will be one reason for the finrot but aggression and improper diet are also contributors. I would not recommend a QT at this point unless the clown developes a heavy bacterial problem. In most cases, improved water quality, high quality enriched foods and eliminating tankmate aggression should clear it up.
Depending on what you feed primarily now, switching to or adding a vitamin booster will help as well as regular weekly water changes to reduce DOC
and other polutants...