What causes the pH to go up (alkaline) in high light tanks?

There are two different mechanisms. One is depletion of CO2. This causes
daily pH swings. As the plants use up the CO2 during the day, the pH climbs.
When the lights go off, the plants stop absorbing CO2, and start releasing it.
The pH drops back down, reaching a low in the morning, just before the lights
go on. A small swing is nothing to worry about, but in high light tanks, the
swing can be large enough to make your fish very unhappy indeed.

The other is "biogenic decalcification," which causes a steady rise over days
or weeks. Not all plants can do this; the ones that can are generally the
plants that only grow submersed. They don't produce aerial leaves, and so have
adapted to getting CO2 out of the water. Egeria and Vallisneria, for example.
If there's not enough CO2, they can extract it from the bicarbonate in the
water. Often, if the plants are doing this, you'll find a chalky precipitate
on the leaves -- calcium carbonate. The KH will drop, the pH will rise. pHs as
high as 10 have been reported, due to biogenic decalcification.

Neither of these mechanisms is a problem with lower light. In low light, the
plants don't grow fast enough to deplete the CO2 in the water. Normal
diffusion with the atmosphere keeps CO2 levels steady, despite the plants'
using some of it.


Leigh

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