what’s wrong with this neon tetra?

[MaloK]

There's betta and barbs in there.

 

[Fishfunn]

I agree with others with it looking like a nipped tail. I think once the bones of a fin are nipped, they dont grow back.

I have a neon tetra with a nipped tail like that. It hasn't gotten any worse, so I leave him alone. His tail has been like that for a year now. I did try treating it with antibiotics, but it didn't cure it. I thought it might be tail rot, but the meds did nothing.