Sounds like your koi may have aeronomas, but you can only find that out by having a bacterial test done by a vet specialized in fish. If it is, then the fish will need to be treated with antibiotics (5 injections spread over a week.) It's not a good idea to treat the pond with a lot of chemicals if you dont know the cause of the illness, it can make matters a lot worse by stressing the fish even more. However the underlying problem is possibly lack of hygene and or too many fish for your filter system. The sediment in the stream (or in the pool for that matter) can be cleaned up with a pond vacuum at regular intervals. Dead plant life should always be removed, rotting things need bacteria to break them down. cleanliness is next to godliness where fish are concerned! Have you checked for things like metals (copper, iron, aluminium are all poison for fish), are you declorinating any water you add, do you make regular water changes? All these things affect fish health. Have you had you fish checked for parasites? I could go on and on, it's no good asking you neighbours for advice, you need to get an expert (Koi vet) to come and look at the pond and find out what is causing the fish to be suceptable to illness. Dont over feed meanwhile as that in it'self cancause problems! Sorry I cant be of more help.

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