Advice regarding being tetra disease

I just had one of my neons die. I suspect neon tetra disease: swimming weird, lethargic, curved spine, kind of pale.

I removed him from my tank before he died and tried to treat on the off chance it was something else but by the end he was laying on the bottom of the quarentine tank gasping so we euthanized him.

The other tank mates are acting normal (tetras and cories). My plan was to move them into a newly planted tank while I converted their existing tank into a planted tank, then move them back to their old tank.

I am not so sure this is a good idea anymore. I'm worried about potentially infecting the new tank. Should I wait a month or so to see if anyone else exhibits symptoms? Longer?

There is a chance he was just old. I have had them since 2020 and who knows how old they were when I got them. No new tank mates have been added in over a year.

Pic of my first attempt at a planted tank for attention.

I just had one of my neons die. I suspect neon tetra disease: swimming weird, lethargic, curved spine, kind of pale.

I removed him from my tank before he died and tried to treat on the off chance it was something else but by the end he was laying on the bottom of the quarentine tank gasping so we euthanized him.

The other tank mates are acting normal (tetras and cories). My plan was to move them into a newly planted tank while I converted their existing tank into a planted tank, then move them back to their old tank.

I am not so sure this is a good idea anymore. I'm worried about potentially infecting the new tank. Should I wait a month or so to see if anyone else exhibits symptoms? Longer?

There is a chance he was just old. I have had them since 2020 and who knows how old they were when I got them. No new tank mates have been added in over a year.

Pic of my first attempt at a planted tank for attention.