One golden tetra death, need help figuring out why (and was it contagious)

Ammonia 0 nitrite 0 nitrate 5 Handful of small plants 10 gal, set up 2 months ago, fishless cycled, fish added ~1 week ago 1 betta, 8 golden tetras, 2 nerite snails Filter rated for 20 gal, set up above the water to create lots of bubbles for o2. There’s so much o2 in here that there’s little bubbles in the water.

All has been well for a week, cycle has been stable with 0,0,5 but I’ve still done 2 water changes during the week too.

All fish fine at 5pm. Around 8pm I see a tetra skimming the surface, gasping. No one else is behaving abnormally. I went to set up a hospital tank, but maybe 30 minutes later he was dead and (oddly) floating at the top. Most dead fish sink.

I didn’t notice any color changes, no redness on the gills, they all ate hungrily and happily yesterday. Tested the water again, params hadn’t changed. No idea what killed this guy.

1 other tetra was then noticed to be slightly tilted horizontally, but it readily ate food and was otherwise behaving normally, and they were schooling to go to sleep at this point so I left him alone. I might’ve just not noticed he has a slight tilt earlier and just noticed now because I was looking so intently. Or this is a new bad sign.

What is y’alls opinion? Did the fish die of a contagious disease, or poor genetics and stress?

Thank you!