04-21-2012, 12:59 PM
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This is the diurnal fluctuation in pH which occurs in nature and in the aquarium, especially one with live plants. It will not be significant (unless something is way out of balance) to harm the fish, as I said it occurs in nature too.
One should always test pH at approximately the same time each day, due to this flucutation. This will allow you to see any changes from day to day.
It has to do with CO2 and carbonic acids, the buffering (KH or Alkalinity) of the water, and the GH (general hardness). Rather than go into all this, have a read of this article which may provide the background; if you have further questons from that, ask away. http://www.tropicalfishkeeping.com/f...quarium-73276/