Neon Tetra Disease

[CanadianWaters]

So today I noticed one my Neon Tetras wasn't looking so good.
He looked: thin, pale/faded and had trouble closing his mouth, doesnt school, as far as I know he is still eating.
Parameters: Ammonia 0, Nitrite 0, Nitrate 10-20
I was worried about NT disease so I currently have him in a 2.5 gallon.
I know there's not a lot you guys can do without a pic but I have him in darkness to help reduce the stress of netting him.
Does it sound like NT disease? Is there anything I can do to help? (It is untreateable right?)
Any help is appreciated!

 

[CanadianWaters]

Well he's not eating but his colors have improved. I am worried if I keep him in a small tank all by himself it might cause more harm than good but I don't want to our him back if he's not healthy.