I Have A Poorly Tetra.

[Snoopy2010uk]

Just got home from a day out and was about to feed the fish when I noticed one of my Neon Tetras looked really ill. I have taken it out and put it on its own for now. What is this and what should I do with the main tank he came out of? Is this something that would happen as they get old?
 
Sorry for the bad photo.

 

[BettaBettas]

water parameters, closer image, tank size, would help thanks!

 

[Snoopy2010uk]

Not done a water check just yet but when iv done them in the past they have always been spot on but will double check soon. its a 100L tank and i'll see what I can do about a better photo.

 

[BettaBettas]

kk