Cory unwell

[Elena82x]

I have a 240l and have had the 8 albino cories in it for nearly 4 years (lost just one a couple of years ago). The last couple of weeks this particular cory has spent more and more time at the surface vertically like in the photo. The last 48hrs nearly the whole time. I’ve checked the water parameters - all fine. The tank is not overstocked. There are two large air stones and a strong filter. The other cories (and all other fish) are acting normally. About 50% of the surface is covered in duckweed. Has anyone come across this before and will it perhaps be an age or illness issue? Visually she looks fine.
Thank you.

 

[GaryE29]

If none of the other fish show signs of illness, that appears to be 'just one of those things that happen'. They're unfortunate and sad, but fish, like us, get hit with illness.