Neon Tetra Disease?

I got 4 zebra danios and 2 cardinal tetras about 1.5 months ago to add to my 29 gallon planted tank.
I just added all zebra danios and 1 healthy cardinal tetra in very recently, but have kept the other tetra in a quarantine tank because it seems to have an illness.
- erratic swimming, uneven
- swimming at a 45 degree angle head up
- I noticed it’s tail is drooping down a little as well
- eating, but not very Interested in food
- has not grown very much at all in 1 month
all the other fish seem healthy, so I added them to my main tank after separating the unhealthy tetra into a small1 gallon tank.
I assumed it was parasites so I treated the 1 gal with general cure, and after the treatment, it got a little better. However, now it has returned to swimming at an angle. No cysts or discolorations so I’m not exactly sure if it neon tetra disease.
thanks for the help!

With neon tetra disease, you will notice discoloration just under the dorsal fin, as it progresses the area will go milky in colour

pictures and video of the fish?

you shouldn't move fish out of quarantine if one is not 100%. however, too late now.

It' not neon disease but there is something wrong.
It could be an internal problem like worms.

Can you post a video of the fish on YouTube and then copy & paste the link here?

Sorry about the long delay, have been busy.
Here is the video.
I've since moved the fish to a larger tank and it has started swimming somewhat horizontally again.

Not neon tetra disease. When putting fish in a QT tank or Isolation give them some plants to hang around in, it will help to lower their stress levels.

It's a weird colour and skinny. It could have something wrong with its insides. I would deworm them and feed it up and see if it improves.

Thanks for the help! Ill post an update on its situation if it gets worse or better.