http://www.monsterfishkeepers.com/forums/showthread.php?299127-Flowerhorn-Mouth-Issue.&
3rd picture in that thread. That fish had swelling in the exact same area as your GT. As you can see, it didn't make it. It couldn't close its mouth(Ducklips usually cause fish to be unable to open their mouths or close their mouths) and started going downhill fast. I'm pretty sure your GT has duck lips. Get him out of that tank and into a hospital tank. Duck lips is highly contagious. Add LOTS of salt into the water and lower the temp to about 75degrees. You could try Methelene Blue or Maracyn/Macaryn 2(both of these methods, not either or).
Great info! I wasnt aware of Ducklips.
I read the link to the thread and the Flowerhon recovered completely. The picture in the first page is from another Flowerhorn.
Hopefully OP's Green terror will make it




