Black skirt tetra looks like a pine cone?????

[Lupin]

It's not just bloating by constipation. It can be bacterial or viral infection as Rich earlier mentioned. It could be intestinal parasites or damaged internal organs. There is no way to tell that unless you dissect the fish which pretty much ruins it anyway.

 

[Kuhlifan]

Definitely get it into a tank by itself. I've had this happen several times in the past and never had a fish survive it yet.

 

[TKOS]

I agree with the QT. As long as the fish seems to be able to live with too much discomfort then keep treating. If it looks to be suffering then I would consider putting it down.