Hello Everybody,

I've got a Diamond Sturgeon (85 cm) in my pond, and I noticed it was not moving or eating a lot.
I've took it out of the pond and put in a quarantine pool with 2 balls to put oxygen in the water.
He almost doesn't move at all, and doesn't respond to food, even when placed in front of him. The food is still untouched after several hours.
You can grab it and move it around without it struggling, which is not normal.
You can see nothing out of the ordinary, no red spots or rubbing or jumping behaviour.
He is very thin (very very) and rarely moves.
It has also barely grown in a year.

EDIT:
There are also two other sturgeons in the pond ( 1 meter) which don't suffer from anything, are very active, and eat a lot. So it doesn't seem to be a water problem (other fishes don't act weird either)

Does anyone have any idea what the problem could be and how to solve it?