Most tap water (at least here in the US) has calcium carbonate wich will
buffer your water.

True, but this wasn't tap water that was added. It was a large volume of
rain water, no buffering. So it could have been a pH crash, but the OP had
apparently had plenty of buffering prior to the rain. ~ jan


I doubt it was a PH crash. I agree that low O2 is
the most likely culprit, fish deaths from low O2 usually occur early in
the morning- before the sun is up.


~Power to the Porg, Flow On!~