Sick Neon Tetra! I’m a beginner and I need advice

[Kuhlikuhli]

I had 7 neon Tetras for about a month. About a week ago one passed very suddenly. He was fine till an hour before and his only issue was swimming weird.
Now another is sick and I can’t figure out what it is.
It’s only swimming strange and it’s eyes seem bigger then the others, so maybe popeye? It also looks like it has some fin rot? They are so small it’s hard for me to tell, so I have to check the others to see if they have finrot too. It also gets bullied out of food so I made sure it got some last night and it’s swimming much better so maybe it doesn’t have enough food? It’s probably a runt as well so I know the odds are against it.
My two main worries and issues is, I’m worried it’s neon terta disease ( but I’m leaning away from that but not sure) and if it’s fun rot and the others have it, how do I treat it. More so because I have kuhli loaches that I love and know they are sensitive to most medicine. So what would be something that can go in with them to help fin rot or should I put the effected tetras in a hospital tank.
My ammonia and nitrite are 0 and nitrate is 10. The water heater stays around 73-76 and I do a 25% water change every Saturday

 

[Colin_T]

It's not neon tetra disease and it doesn't appear to have fin rot.

It looks skinny and stressed and the damaged fin is probably from being picked on by the others.

------------
Check the water quality for ammonia, nitrite, nitrate, pH and GH.

Do a 75% water change and gravel clean the substrate.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Add 2 heaped tablespoons of rock salt (aquarium salt) for every 20 litres/ 5 gallons of tank water. Keep the salt in there for 2 weeks.

Monitor the fish and see how they do over the next 2 weeks. If more of them get skinny and die, post pictures. They might have gill flukes and intestinal worms. The salt will kill any gill flukes but you might need a deworming medication if they don't improve over the next 2 weeks.