Another update:

A bowl of pond water which had just done in a goldfish was taken
inside the house and allowed to equilibrate to the same temp as the
other bowl of water containing kitchen faucet water. I was concerned
that the elevated temp of the outside pond water was responsible for
killing fish. I took one of the goldfish that had been living in the
kitchen faucet water for over 24hrs, and placed it in the
temp-equilibrated pond water. I stayed up for a couple of hours, and
although the fish had not died, it was clearly not doing well when I
went to bed. It was dead in the morning, while the fish in the bowl
with kitchen faucet water were fine.

After all the tests and dead goldfish, I believe that I have
determined a possible cause. When adding a large trash bag filled
with ice to cool the pond, I caused considerable waves in the pond. I
noticed that there was a small puddle next to the pond which moved in
sync with the pond water disturbance. I believe that there is a leak
in the pond somewhere (difficult to tell with this black preformed
liner) which is in equilibrium with water which has collected under
the pond. The water under the pond must have some toxic substance in
it which is contaminating the pond. This conclusion makes sense to me
since water taken directly from the auto-refill does not kill fish,
but water coming from the auto-refill system into a freshly cleaned
pond liner does quickly kill fish.

As far as the pond size, I believe I was incorrect in my original
posting that the pond is about 150 gallons. A landscaper installed
the pond, so I don't have the documentation on it. I went to the pond
liner manufacturer site and I believe I found the liner that matches
our shape and it is 250 gallons.

I plan to completely empty the pond to dryness, and thoroughly inspect
the pond liner to see if I can find any leaks.