Photosynthesis consumes CO2
and releases O2
during the day. At night, O2
is consumed but CO2
is not, so it builds up. Accumulated CO2
in the water causes pH to drop at night, which is why it's usually lowest in the morning and highest at the end of the day.
A side note about test kits... high levels of ammonia and nitrite interfere with the chemicals used during testing, so testing anything but ammonia, nitrite, and nitrate during the cycle is pretty much not needed at all, unless you just want the practice.
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