Poorly tetra

[Catfish586]

hi yesterday I noticed that a black neon tetra looked painfully thin, almost as if his stomach area had wasted away. This morning I found him floating - presumably deceased - and fished him out, but he started twitching. I've isolated him with a newt and fed him, and he is floating normally but still looks very weak. Is there anything I can do or would euthanasia with clove oil or bicarb be the kindest thing?
I will test the water and do a partial change etc, although I replaced 25% less than a week ago and the other fish are fine

 

[Colin_T]

pictures and video of sick fish?

if it can't swim and has lost weight suddenly, euthanise it.

 

[Catfish586]

Centre of video.... just painfully thin

 

[Colin_T]

euthanise it

 

[Catfish586]

done, second loss in recent weeks