This is my little guy
View attachment 296650
This picture (and my next statement is based solely on it)
Copied in from elsewhere, just trying to help!
"Skinny Disease (clown loach can be prone to this)
The disease is caused by a bacterial infection that causes the fish to have a sucked-in gill appearance. Its head will often appear much bigger than the rest of its body.
Treating Skinny Disease: Adding extra food to the fish’s diet can usually clear up this disease. However, sometimes this doesn’t help and if the bacterial infection persists, adding erythromycin to the fish’s food normally clears the infection up quickly."
"Chilodonella: Some of the symptoms included with this protozoan parasite are: dying on the surface and rolling over on the fish’s s sides. This is a notorious killer of koi.
Treating Chilodonella: You will want to increase the salt concentration in the pond for 2 weeks as well as increasing aeration.
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What is considered to be a normal salt concentration for koi?
(I thought they were freshwater fish)
What type of salt are you using?
What is your concentration/administration method?
Can you get any more pictures, particularly the gill site (each side)
An overall side shot, a top shot, abdominal area (pectoral to anal fin)
They may be of some use if the above isn't any help at all!
If the fish is still alive, there is hopefully time to diagnose it properly.
I don't know much about koi but most ill health is the same with only a few species specific diseases.
You should go slowly, target each suspect individually, and complete fully the recommended course of action. Be careful mixing treatment.
Edit.....
Just a thought,
What happens when the fish is not in the net?