Mature Neon Tetra, swim bladder disease?

I have a 10G tank with 5 black-fin tetras and 2 neons. The tank is
about 2 years old, and so are the neons, but the black-fin tetras were
added about 6 months ago. I also added 2 ottos 6 months ago; one died
within a few weeks, and the other just passed today (I'll get to that
shortly). There are no live plants.

A few days ago, I noticed that one of my neons is acting funny. He's
sort of bowed in the middle in a U shape; well, like a flattened-out U,
anyway. He also swimming at about a 45 degree angle, and his head is
higher than his tail.

I thought he might be constipated (had this problem with a now-passed
betta once) so I held off feeding him for a day, but that had no
impact. He's still just as shiny as usual, there are no oddities on his
body, and he ate just fine the next day. He's schooling with the other
neon like usual, just sideways and you can tell that he's paddling a
lot harder than usual.

Although I haven't seen him poop, let me say that in 2 years, I've
NEVER seen the neons poop, so that's not unusual.

Absolutely nothing in my tank has changed in 6 months. I do 10% water
changes each Saturday, and feed them Tetra flakes once a day. The
filter needs to be replaced, but I'm not sure about the correct way to
do that and I'm afraid that doing so now might cause bigger problems.
Nitrites are at 0, and the pH is at 7.2. Temperature is a fairly
consistent 76 (goes down to 74 at night).

I haven't been able to find a lot of diagnostic information for neons,
and I don't have much experience in successfully nursing a sick fish,
but I'm concerned that this may be a contagious infection, or dropsy.
My other tetras are acting normal.

The day that I skipped a feeding, I also did a 25% water change and
good gravel vacuuming. 2 days later (today), my other otto passed for
what appears to be no reason. The other fish are still fine.

And finally, I don't have a hospital tank to quarantine my sick neon. I
could put him in an unfiltered 1/2G bowl for a short period of time,
but that's about all I have to work with.

What do you all think? Does this sound like an internal infection or
dropsy? If so, what's the best way that I can treat him? Are my other
fish in danger?

Thanks,

J