Judging by what you have said I would definately say it's the swim bladder. It can be sometimes from constapation beleive it or not.
Have you seen it poop lately? If not try feeding some green peas. That should "clean the pipes" and reduce the pressure on the bladder. If that does it then it's all good, I would try that first..
If it's diet is producing the proper waste then you should QT the regal and treat for possible swim bladder infection. Soaking the foods in a gram negative antibiotic like Maracyn II is best. Pepare the dosage in a glass of tank water and take out a few teaspoons and place it in a small cup with some brine add the remaining amount to the QT. Do this for the first day or two. After that just continue for seven days with the normal addition of antibiotic. Do not
treat the antibiotic in the main tank only in a QT environment. Maracyn II contains a vitamin additive so the other vitamin additions are not really necessary.
The agression is normal. Be glad it's not the other way around. How big is the regal?
For future reference... Zoe
, Something that you should consider adding to their dietary menu.