I'd agree with that totally, but the tank that orginated this new thread was not a planted tank, as we would see it! It had a few killis and, I think a bit of Java moss!
I think his fish were killed by a brief ammonia spike, or some other pollutant, and then this subsided before testing the water. But, as it was treated by the filter, this produced acid that also caused the pH to drop.
What do you reckon? Possible? Probable? ???