Koi Dying After Heavy Rainfall

Question
QUESTION: I recently added 5 small koi to a very large koi pond that currently has Koi
that are 5 years old and much larger.  All 5 of the new Koi added have dyed
after going through quarantine and then each of them dying over a period of
a week in the pond.  My real concern is now I have lost 4 large koi with no
apparent injuries or viewable problems. They look healthy and showed no
signs of illness.  We have been experiencing heavy rains and I tested the
water and everything looks okay.  In fact the water is more clear now than it
has been all season.  Any ideas on what may be killing my Koi?  I am totally at
a loss because there are no signs just dead koi. Please help..

ANSWER: Hi Steve,
Thanx for your question.  Can you tell me your general location?  Have there been extremes in temperature?  Are there any trees or plants around the pond that are dropping leaves, flowers, seeds, nuts, etc., into the pond?  Any run off from the surrounding soil getting in the pond?  Let me know so I can best answer your question.
thx,
Tom

---------- FOLLOW-UP ----------

QUESTION: Columbia MO - There have been extremes: 90+ daytime temps  The pond
is
elevated to avoid runoff.  It is surrounded by trees: Elm, Hickory, and Ash.  I
also have a couple of hummingbird feeders close by.  At this point I have lost
all Koi except 1 older Koi (3 years old) that has been in the pond the whole
time.  No goldfish have been affected.  Water conditions check out normal.  
Could it be KHV?  I'm wondering what to do next to basically sterilize things
ta this point.  i could move the goldfish and one remaining Koi to another
pond.  I'm at a loss to figure it out.  The Koi have died in order of size and
age.  youngest/smallest first gradually to the oldest 4 years old.  I have salted
the water but other than a water change and salt after it started I have not
treated.  I would like to think about re-stocking but you can imagine my
reluctance thinking something may still be present.  Thanks for any advice.

Answer
Hi Steve,
Thanx for your question.  Sometimes after heavy rains, there can be fish kills.  I have never had this happen to me, only read about it.  I can't tell you if it is KHV.  Here is a link you can use to get more information about the disease. http://edis.ifas.ufl.edu/VM113.  Do you have an aerator in the pond?  Hot weather can cause oxygen depletion which will cause the fish to die.  I can't think of anything else.  You might want to consult with a local nursery that has an ornamental pond specialty.  I hope this helps.
Tom