The graph looks like it came from an apex? The x axis is time. Not sure what the scale is...looks like each tick is a day given the cyclic nature of the pH.

Feeding kalkwasser with your top off help elevate your pH overall. Dripping it at night only will reduce the difference between peaks and valleys.

Running a fuge will help raise your pH also because the photosynthesis consumes CO2. Running lights on the fuge in a reverse photocycle will help reduce the difference between peaks and valleys.

Probably the best case would be feeding kalk with top off and running a fuge with reverse light cycles. This will elevate and help to reduce the peaks and valleys. I'm not saying this will fix it 100% but it should help if you aren't already doing it.

Move the effluent of your calcium reactor to your fuge. The algae will help uptake CO2 before getting into the rest of the system.

^Note these suggestions will be more effective with a larger size fuge.

Other possible suggestions that come to mind:

I have read about people that have connected an air line on their skimmer venturi and run it outside to get fresh air to help with the problem. I imagine you can only run it so long though or their won't be enough force to get air into the skimmer...could always hook up an air pump I suppose to give it a boost.

Try increasing surface aggitation in the display or sump. This could help or hurt depending on whether the controlling factor for elevated CO2 is the indoor air or excessive CO2 in the tank. Either way, could be worth a try to see whether pH goes up or down as a result.

I eventually gave up on my calcium reactor due to the same problems you are having. With the the relatively small fuge I had, it wasn't really enough to treat the CO2. I had a korallin calcium reactor which did not have the second chamber. I'm not sure what reactor you have? I suspect a two chamber unit would be easier to regulate excess CO2. (BTW- When you get a chance, start a reef showcase so your system specs are linked in your postbit).