Diagnos this Neon Ailment

I'd say it's most likely bacterial...could even be NTDwith a secondary bacterial infection, but whatever it is, I hope you've gotten the fish out of your main tank? What are you feeding them? Any live foods? Are any of your other fish showing any type of "different" behavior?

If it were me, I would do some daily water changes of at least 25%. NTD is more common than people may realize. Some good maintenance and vac'ing will help rid the tank of the majority of the spores. (if it actually is NTD) If it's bacterial, this is usually a result of water quality. Are the tetras a new addition? They could have been ill prior to purchase.

I also instantly suspected the NTD. No treatment as far as I'm aware. Seclude the fish, and try medications. I've had a similar problem with a fish a couple of years ago, and it died from starvation. Best of luck!

LOL geez I sorta knew it would be diagnosed as NTD. Seems like everything wrong that happens to neons is considered NTD.

Doesn't sound like NTD to me. NTD is characterized by cramped looking muscles all over the fish. I could be wrong but keep an eye on your other characins in case I am. All characins are susceptible to NTD. I'd almost bet on it simply being a bacterial infection only.

NTD so so far incurable with no known remedies. I'm lucky I havent come across it (knock onwood), but it spreads like a mofo and you will want to seclude that fish and wait it out (untill it dies) or you could cull it if you feel bad of its suffering. The only thing that im suspect about it maybe NOT being NTD is that you havent said that the body is all bent and stuff. Usually NTD causes the spine to go all bendy and stuff.