[blue eyes]

this week i have lost two of my israelie koi for no apparant reason.the pond has been running for 3 years now and i have never lost a fish before.the other fish are fine.there were no signs of injury or disease,they simply just died.
the water is fine and healthy.
is this just a case of when their time is up that's it?
my friend has kept koi for 13 years and he cannot see any reason either for their sudden demise.
is there anything i could add to the water just in case

[bcfc]

dont put anythink in ur water yet, tell us ur water test , ph, amm, nitrite, kh?? have u done a scrape ??