Cna't make much out in the pic. Is it a parasite or fin bone thing.
Any chance of a photograph from above.
Can you list tank size and other occupants please.
What decorations are in the tank??
Has another fish been nibbling at him.
I would post in tropical discussion aswell as not sure what to suggest doing.
Yours tanks overstocked.
Can you post your water stats please in ammonia, nitrite, nitrate, and ph.
Has the bone just started showing overnight, or has the skin been peeling away for a while.
I would do a water change for now.
Take a sample of your water to the lfs and ask them to write the readings down for you.
I would strongly suggest investing in some liquid test kits.
Check his body for peeling skin, Are his fins ok, are they rotting away.
Bad water quality can make the skin peel away.
Any signs of flicking and rubbing.
Do you have any melafix as I would just add that to stop infection.
A api master kits about 18 pounds I think.
Add the tetra med then if its for external wounds.
Get your water tested.