04-23-2012, 12:47 PM
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Agree. To answer your question in general terms, the pH of water in the aquarium can fluctuate depending upon several factors. This may or may not be good, depending upon the numbers of the tap water and tank water and the fish species. And fish can die from sudden or drastic pH changes.
To properly assess the issue we need to know the GH (general hardness) and KH (carbonate or Alkalinity) and pH of the tap water (presumably your source water), and the pH of the tank water. You can read about the correlation between hardness and pH here: Water Hardness and pH in the Freshwater Aquarium
Also, since fish deaths are occurring, have you tested ammonia, nitrite and nitrate? What size is the tank, and what fish are in it? How long has it been running? Any live plants? And what water temperature?