It's normal for the ph of a planted tank to increase dramatically while the
lights are on if you are not adding co2. Check it again after the lights
have been out all night right before they come on again.

I am not 100% sure of all of the causes, but here one contributing factor
is the plants are using up all of the co2 decreasing the buffering capacity
and increasing the ph slightly.

also when the plants run out of co2 they start breaking down bicarbonate to
get the carbon they need and this also raises the ph.

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Margolis
http://web.archive.org/web/200302152...qs/AGQ2FAQ.htm
http://www.unrealtower.org/faq