Fish dying

In the last month, I've found two of my goldfish in my outdoor pond
dead. The water chemistry is fine except for the hardness being too
low. I added a bunch of dolomite lime almost a month ago after the
first fish died, and the hardness is improving, although the pH today
was still on the low side. I don't know of any toxins that could
have gotten into the water, and the fish showed no obvious sign of
injury. Is there something else I should be doing or thinking about
to protect my remaining fish?

Thanks.

Joan
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