Unless your Ca is running really high, I think you need to increase the output of your reactor a bit. Low Alk allows for a larger daily pH swing. (Isn't the definition of Alkalinity the resistance to pH swing? - buffering?)
If your reactor is already running at 6.5, increase the flow a tiny bit.
Measure the Alk right before your lights come on. That should be it's lowest level point and will give you a good idea of how low it's going.
Yes, you could add a Kalk reactor...and run it only at night. The trick there is that it is your Ca/Alk levels that determine how much Kalk you can add. You probably can't add enough to make any difference to that cycle unless you turn off your Ca Reactor.
The bottom line is that the cycle you describe is pretty normal...especially for a Ca Reactor system with no refugium. Don't do anything too radical to try to fix it.
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank: http://www.canreef.com/vbulletin/showthread.php?t=28436
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