Can you give us some more info?
What is the pond temperature?
Have the fish been being fed?
What are the numbers for ammonia, nitrite, pH, KH, nitrate?
This time of year is particularly hard on koi, since they are just waking up
from a long winters nap, their immune system is very low, parasites get a
head start, and bad bacteria can thrive at the lower temperatures.
With more info, I am sure you will get several possible ideas of what
happened, and what to do about it.
--
RichToyBox
http://www.geocities.com/richtoybox/pondintro.html

"colin smith" wrote in message
. uk...
One of my Koi has died ,it became lethargic I put it into isolation with
some treatment for fungal bacterial ect diseases and then returned to main
pond.After a few days it died there was no external indication of any
problems other than its behaviour.I have tested the water and its OK. I am
very new to keeping fish and I am worried that the other fish will be
affected any advice would be appreciated.