then approximately 4pm or so is when you should check it. If pH swings that much, you might want to test your alkalinity levels. Alkalinity is the measure of the ability for pH to stabilize. The higher your alkalinity, the better your pH is stabilized.
Use a pH buffer in some top off water. Then, test your water for pH and alkalinity. the dkH test should read from 8 to 12dkh. Anything less and there is a potential problem. If you are going to add baking soda, you should use a half a table spoon per 10 gallons of aquarium volume to raise alkalinity by 1 dkH. *(thank you bangguy)*
When you get your pH raised, and your alkalinity up to at least 8dkH, you can start dripping kalkwasser. "lime water" will both raise your calcium and alkalinity in equil parts. Just make sure you dose it on a drip. I generally like to drip 2 times per second throughout the day. I have a 5 gallon kalk drip on my 240. (I drip every day except Sunday, and on Sundays I add a pH buffer to make up for the lost levels of ca and alk.)
Anyways, hope this helps.
