When all the fish die overnight in a tank,
I usually suspect a poisoning, or a heater malfunction.
You said your pH was 8, but was it only 8?
Some of the chemicals available for buffering,
such as Seachem Tanganyika buffer and Kent Cichlid buffer,
can be TOO strong when over-used.
I know of one case where a woman used a double dose of one of these, then did a pH reading with an electronic pen, and got a pH reading of 11. None of the fish died, but all were stressed.
She did a 75% water change, the pH went down to 9 and then all the fish were fine. I'd recommend using the Tetra pH test that reads
from 5-10 to get a better general idea of your pH.
Other poisons can be any of a hundred household chemicals.
Soap takes about 24 to poison fish. Windex, Raid, air freshener,
a cigarette butt floating in the tank, bitter apple, and probably
dozens of other items I can't think of can all kill fish in tiny ammounts.
_________________
Ned
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