I have "heard" somewhere that butterfly are supposed to be more hardy.
Cant say if that is really true or not. Have not had a sick butterfly
yet for what ever that is worth.

What are the circumstances of their death? Any signs of trauma or
disease? Any new fish added. Where they the bigest fish or not?

Sorry you had to loose fish.
Lisa wrote:
I have lost two butterfly koi in the last week.My other two koi are
doing well and have great appetites.There are no signs of injury or
disease in either fish.If anyone has a answer please let me know?