If you look at the fish from above, does it look like a pine cone (scales are open)? Or is just his stomach a bit thicker? In the latter case, it can also just be a female that comes into laying.
Dropsy can be due to a virus or bacteria, and then they are contagious, but this is rare.
Much more often your fish gets dropsy because its health is so bad that it can no longer pump enough water out of its body. Once a fish has dropsy it is unlikely that it will get through. You can try to set it aside in a container where you added salt to the water. It is best to start with 2 to 3 g of salt per liter of water (pond salt).
But the most important thing is to find out what caused it to prevent the other fish from getting sick too.
Start by getting your water tested.
The ideal water values for a koi pond are:
- Ammonia 0 (NH3)
- Nitrite 0 (NO2)
- Nitrate as low as possible (NO3)
- PH 7.5 – 8.
- GH Value 8-12
- KH value 5-8
If deviations are measurable in the above list, one must respond immediately in a number of cases.
If that’s okay, observe the other fish as well. Do you see them scouring or flashing through the water? Do they often gasp for air on the surface? Do any of your fish have wounds? Then it could be that they have parasites.
If you don’t see anything special about the other fish or the water tests, it may be that that one fish has just had something to offer. And it stays with that one fish.