That's interesting, but did the pH go down? CO2 dissolved in water lowers the pH.This used to happen to me during the winter. I would wake up and several fish dead overnight with no signs of illness. It seems excessive CO2 in water was the cause. During cold months no open windows or doors so it builds up. I placed an air pump outside a window and ran a long airline hose to the tank for a few hours a day.
This stopped the mystery deaths for me.


