Large Boesemani Rainbowfish sunken belly and discolouration

[danj_91]

One of my older Boesemani Rainbowfish has what appears to be a sunken belly directly below its head. It has lost alot of its colouration compared to others in the tank.
The fish is still feeding but is being bullied now even by smaller rainbowfish.
Breathing doesn't look any faster than normal, however the fish is spending most of its time in the upper levels of the water column and isn't overly active unless being chased.
I've added a few photos to help with diagnosis.
I'd really like to save this fish if possible as I've had it for a long time and it's now reached a fairly substantial size.

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