11-24-2008, 07:24 AM
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pH swings from water that is left out is largely due to outgassing and what ever is going to happen because of it is usually over within 24 hours. Daily pH swings are common on planted tanks as during the night when they aren't photosynthizising they release carbon dioxide rather than absorb it. In a lowlight tank the plants are likely not utilizing much of the CO2 in the water and therefore the swings are likely much lower, low enough to be difficult to detect by conventional means.