If this is your fish(which I doubt due to your habit of posting other peoples images as your own) my guess would be water quality problems. Also as far as it being kept alone if that's so then why the divider? Why don't you post a full tank shot so we can see what's on the other side of the divider in "your" flowerhorns tank, never mind the fact that you've posted your freshwater fish in the Marine and Brackish section. Which I think you might've done because you've been recognized in some of the other sections.