Advice needed - pond PH too high, my koi died

I'm new to rec.ponds and to pond keeping in general, and I hope you
more experienced people can help me with a problem I have with my
pond.

My pond is very small (about 50 gallons or so I think) and I had two
cute little 3" koi. They lived for about a month. When the second
died today, I took a sample of water in to where I bought the koi and
they tested it, and told me it was too acidic. The other levels were
OK. They recommended a chemical treatment called "PH Down" but,
because I have two dogs that think nothing of drinking from the pond,
I don't want to use that product as it's hazardous. Any other ideas
out there on how to bring down the PH of my pond, in a way that would
be safe for pond-drinking pets? Also, any ideas on why this happened
and how I can avoid it in the future? If it is helpful to know, I
also have in this pond 3 aquatic plants (the kind you put in
aquariums), a "sucker fish" to clean up the sides of the pond, two
snails and a flag lily. There are two small pumps drawing water
through filters and one of them sends the water through a spitter to
help with aerating it. I had just done a forced water change of about
half the pond water in the week before the last koi died; one of the
pumps got bumped and pumped half the pond water out into the garden.
I replaced the water over several days; each batch of water I added
to the pond sat out 24 hours before adding it. It also rained heavily
the day before the last fish died.

I feel so bad about the koi and don't want to get any more unless I
can keep them healthy! Thank you for any advice you can give me.