Swim bladder problems are not as common in Koi as in fancy goldfish.

I lost a fish with a bladder infection that was not apparent until I did the post mortem. the swim bladder was half the size of the fish's abdomen and as thick as a marigold washing-up glove. It must have taken a long while to develop. The fish was an eight year old 8Kg mirror carp. It's belly was as tight as a drum and it showed itself in last years hot summer. The fish could not leave the surface.
My fish mean a lot to me and putting the fish to sleep took a lot of doing.
The point I'm getting to is swim bladder problems are many fold and swim bladder treatments for a problem that could have upwards of 20 causes seems to me a waste of time.
Most swim bladder problems show them self in the spring or the onset of cold weather.
Warm salty water at 20°C may help.
One reason could be constipation. Alternatively the fish may have ovarian tumour or infected impacted egg mass. This would look like your fattest fish but in fact would have no reserve fat and is organically emaciated, with next to no liver mass etc. The walls of the uterus could have tumours.
It could also be due to cystic kidneys. These are large yellow through to orange fluid filled sacks that arise from the caudal pole of the kidney.

The list goes on.
So swim bladder problems in fish keeping terms looks like a single symptom but with many causes.