"*muffin*" wrote in message ...
I added 2 more pics.

It looks like something is rotting in the pond. Did perhaps an animal
fall in and drown? You might check the bottom to see if anything is
there. Is there an excess of vegetation? It doesn't look like motor
oil.

I would try to catch out the koi, perhaps by luring them with food to
a pre sunk net, then when they are eating you can quickly raise the
net and get them. Then treat them in a hospital tank. I don't like the
look of those lesions. I think concentrated antibiotic is called for.

good luck!