Last year I lost 10 to 15 large 9 year old KOI fish in June just after 
spawning.  I caught them when they were swimming on their side and poured CS in 
their mouth.  All sick KOIs died.  I have around 100 KOI in my pond from 9 year 
olds to last year babies.  All the other fish seemed to be OK.
Brickey