Black Widow Tetra - looking poorly

Hello
I hope you are well
Is anyone able to advise on what might be wrong with this black widow Tetra.
80l well established (years) with handful (4) albino cory cat fish and two other Tetra.
No other fish are affected, tank is changed every 10 days - two weeks at 50 - 75% change.
Prime is used as conditioner.
Tetra is hanging around the top of the tank, swollen, and either stuff is coming out or it's been attacked?
Any advice gratefully received
Kind regards
Sarah

Hard to tell from the pictures.
How long did it take to get fat?
Is it eating normally?
What does its poop look like?

Is it swimming normally or does it float or sink when it stops swimming?
Is it breathing heavily/ rapidly?

If it got fat overnight and isn't eating, then it has an internal bacterial infection and will probably be dead in a day or two.

Internal bacterial infections normally cause the fish to breath rapidly either near a filter outlet or at the surface. If it is breathing heavily, increase aeration/ surface turbulence to maximise the oxygen level in the water.

Thank you for your reply, it's been like it for three days.
It doesn't appear to be breathing heavy at all, just normal but constantly at the top.
There is a fast air pump operating already and the filter pipe is above the water slightly so causing some bubbles and movement. I am able to add a second air pump?
It's swimming as normally as it can whilst arching towards the surface.
It's not going for any food though.

Can you post a 1 minute video of it?
You can upload videos to YouTube, then copy & paste the link here.
If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the entire screen and doesn't have black bars on either end.

Try to show the entire tank too so I can check the aeration.

The fish is dying, presumably from an internal bacterial infection. There's no cure at this stage so euthanise it.

Thank you.
Is there anything else that I need to do to protect the other fish?

You can do a big (75%) water change and gravel clean the substrate every day or every second day for a week after a fish dies. This dilutes disease organisms in the tank and reduces the chance of others getting sick. Give the filter a clean too.

Apart from that there isn't much you can do to help or stop these types of issues. Internal bacterial infections can originate from the breeders, suppliers, pet shop or the food you give the fish (food is rare and it normally occurs at the breeders), and you can buy healthy looking fish that can die from infections or organ failure months after you get them. There's no way of telling if a fish has an infection or is prone to an infection, or has a genetic predisposition for organ failure until it happens. It's just a matter of doing regular maintenance, feeding a varied diet, reducing stress and hope the fish are ok.

If another fish develops the same symptoms, let us know asap but internal infections and organ failure don't normally spread from fish to fish unless it's Fish TB.