algae/fish kill

We had an entire day of heavy rain on Friday. My sisters pond which is
about 500g is located in full sun, raised above ground abour 1/2 a
foot, and usually has green algae hiding the 8 fish.

Saturday morning we came out to find the pond completely crystal
clear....and then noticed the fish were all just sort of hanging in the
water....or on the bottom. There was no ammonia level and the ph was
8.0. She tried to rescue the fish to a separate tank, but only two
have survived.

Any idea what could have caused this massive die off? The only variable
we can come up with was the rain.... She doesn't use any chemicals on
her property or lawn. As soon as I saw that the algae was gone, I
thought this couldn't be good, at first thinking the fish were hiding
due to lack of cover. I just lost a frog after this rain, and my
neighbors largest fish died.

Thanks for any input, the fish looked fine, no fuzz/fungus/weird
looking things there, and were fine on Thursday.

Mo