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.

How is your water treated, by whom? Some municipalities have stopped
using chlorine to treat domestic water and have started using things like
ozone, bromine partly to improve the taste and partly because they have
longer half lives and are considered more effective. Check with your
water supplier.

How are you refilling the pond? If you are using the automatic re-filler
mentioned above try filling from some other source. Use the kitchen tap
and a hose. I only suggest this as your problems started when you turned
the automatic system back on.

If the water is still being chlorinated the a test for this might be to
fill a small container with tap water, either let it sit for 24Hrs or add
de-chlorinator and introduce a small fish. If the fish survives try the
same thing using the water source you are using to fill the pond. Take
the sample at the point it enters the pond, but don't take it from the
pond. If the fish survives this part of the test then the problem is in
the pond, if it doesn't then the problem is in the water used to fill the
pond.

WARNING, If the fish doesn't survive part one of the test GET YOUR WATER
TESTED YOU MAY BE AT RISK!