See this . pH changes during the day.

from SRAC
Fish and other vertebrates have an
average blood pH of 7.4. Fish
blood comes into close contact
with water (1- or 2-cell separation)
as it passes through the blood vessels
of the gills and skin. A desirable
range for pond water pH
would be close to that of fish
blood (i.e., 7.0 to 8.0). Fish may become
stressed and die if the pH
drops below 5 (e.g., acidic runoff)
or rises above 10 (e.g., low alkalinity
combined with intense photosynthesis
by dense algal blooms –
phytoplankton or filamentous
algae).
Pond pH varies throughout the
day due to respiration and photosynthesis

[Linked Image from forums.pondboss.com]