[Roy hit the nail on the head, large fish dying first, usually means low O2.

I would recommend those who live in areas of much rainfall have the test to
measure KH. Rain water has no buffering, and supplemental treatments need
to be added unless other means are in the ponds.

Okay, help me out RPers, was it limestone or crush oyster shells that was
recommended as a slower method than baking soda to increase buffering?
~ jan


~Power to the Porg, Flow On!~[/quote]

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