Poorly ember

[Catfish586]

I've got a poorly small ember tetra which was among a recent influx a few weeks ago. It's separated itself from the rest, has no orange/colour whatsoever, is visibly poorly and its stomach seems distended/swollen, particularly at one side, and has started to look black in the last day or so. He's been like this for a few days and is clearly a fighter, but he's not looking good. Anyone know what the problem might be and whether it's treatable? I lost another of the newbies a fortnight ago. Many thanks