Simply Koi and More Koi

checked water yesterday all fine done water change today and salted pond need to do a scrape waiting for a microscope off my mate will do this later when he gets home from work any other surgestions ty

over the last few days i have lost bout 10 koi i have checked them best i can no visable marks or spots gills look ok checked nitrite which is fine any ideas what i can do next as no clue spoke to my mate kim she said ask here any help would be gratefully recieved dont want no more dead ones cheers ...