Help! Why is my tetra exploding?

[Hannahclairise]

This neon has had a lumpy abdomen for months. At first I was worried, especially when the window for pregnancy passed and it was still bloated. I eventually stopped worrying because it otherwise seemed fine- coloring was great, still ate, swam normally etc.
5 days ago I transferred my school of 6 small neon from an established 5 gallon to my new 20-gallon tank. They all seem super happy and have been schooling together and the lumpy one seemed totally normal. Then last night I noticed that his abdomen had burst, and pink gunk is coming out. I immediately removed him and transferred to baggie in the tank to maintain temp. I decided not to immediately euthanize as STILL he seemed totally fine otherwise, and I'm curious to see what happens in case any of the others become infected. This morning he's still fine- excepting the disgusting, pink, sloughing explosion. What is it, and what should I do??

Parameters: Have had the tank set up for 4 weeks, seeded from established tank.Having the dreaded not-quite-zero ammonia reading with api test. Have been low-dosing with prime since adding fish just in case, as well as every other day 25% water changes and only feeding every other day. Nitrites 0ppm, nitrates 15ppm. Temp 77 degrees.
Filtratation: aqueon quiet flow 30 with added foam pre-filter.
Stocking: 6 small neon tetras (all under 1"), one baby albino bristlenose pleco. Lots of plants.

[Aiken Drum]

White cysts are one of the symptoms of neon tetra disease. If you cant isolate the fish properly i would euthanise. Do you still have that 5g?

[Hannahclairise]

I do, but it's home to a school of rasboras now and I don't want to risk them. Thanks.

[Danio8]

Definitely don't risk it. You tetra might have "Neon Tetra Disease". It can infect Rasboras as well. Might find this website helpful. https://www.thesprucepets.com/neon-t...isease-1378484

[Hannahclairise]

I put the poor guy down today as I don't have a good way to quarantine and I don't want to risk further exposure for the others. I'm really hoping it wasn't NTD. A couple of things make me skeptical 1) he lived for a while with whatever was going on. It was my understanding that NTD is a rapid killer. 2) other than the mysterious bump, he seemed normal and happy, not wasting away at all.
I'll keep an eye on the others and avoid adding more fish for a couple more weeks to be sure.