Hello everyone, can somebody help me please....I am relly worried.

I have a twenty gallon tank, 6 months old.
I have 9 neon tetras. Only one of them looks like he went into a blender...is he going to die? or spread to others? he is in really bad shape...I have been watching carefully for bullying but no one seems to be bothering the Neon tetras. and he is the only one that looks bad.
I already treated my tank parasites and fungus but I do not see improvement. I did two weeks treatments of each fungus and parasites. Rest two weeks in between with carbon filter and proper water changes.

What happened to my neon tetra? I have not had fish loss since I set up my tank. He might be the first one. I have read that Neon Tetras are hard to keep...