Thomas McMillan said:
Just to clarify, I wasn't suggesting you just buy some random treatment, dose it and hope that it works. That's why I said I wasn't really sure what to do and to go to Maidenhead for further advice on that.
I'm sorry but it seemed like that was exactly what you were doing. Often the guys in unfortunately far too many LFSs will sell you something on the chance it might help and to make a sale when actually what you need to do is sort out the water quality and then leave well alone. You should never dose unless you are sure the treatment will actually be effective against the diagnosed problem. Koi keeping IMHO is far ahead of the rest of fish keeping here where scrapes and microscopes are more widely used to diagnose parasitic and other issues. You can then be relatively sure of what's wrong with a fish and dose with a powerful, fairly specific drug that sorts the problem out.