Fish pond water kills all fish within 24 hours.

I have a 150 gallon outdoor pond. It was populated by 3 koi, and a
small catfish for over a year and a half until the automatic refill
was inadvertantly turned off. When it was turned back on the fish
died the next day. I assumed it was from a large influx of
chlorinated water. Anyway, I took the opportunity to totally clean
out the pond including about a inch of sludge, and small rocks on the
bottom. New water was added, dechlorinator/conditioning solution
added, and the pond was left to circulate for 48 hours. New koi were
added (after acclimation) and died within 1 hour. I changed the water
3 complete times, reconditioned the water, let it circulate for 1
week, added some comets which died within two hours. I changed the
water again three times, let it circulate for 3 weeks, took a water
sample to my local LFS. They told me that the ammonia was slightly
high (but not deadly) pH, was about 8.1 (slightly high but not
deadly). Basically they told me that they did not know why the fish
died. Using the same tap water that the pond is refilled from, I
placed some comets which lived for a week without problem in a bowl in
my home. When placed the comets in the pond, after acclimation they
died within a day. WHAT IS GOING ON? It appears that there is
something quite toxic to fish in the fish pond. Still I have changed
out the water multiple multiple times with no improvement. The toxic
substance must be specific to the pond, since the tap water did not
kill the fish in a fish bowl. The only things I have in the pond are
two pumps (one is a new fountain pump (Beckett) and the other is the
waterfall pump which has been there for two years without problem.
The pumps are on GFI circuits so I don't think there could be a large
short circuit. There are Lava type rocks in the pond which, once
again were in there with the koi for over a year without problem. I
did power wash the rocks prior to putting them back in the pond. The
water temp is in the low 80's. The water is perfectly clear. Please
any help appreciated since I am at a loss what to do next.