Maintaining pH

Running a lit refugium opposite the main lighting cycle will help.

I'm running into this challenge also, I tried opposite light for my sump but only brought the ph up for an hour the dropped down again.  I tried doing the top off with kalk in the middle of the night - I didn't even notice a change in the PH.  

My 50 cube is a SPS dominate tank, and I run a Ca reactor (built by tcfab) and it keeps my pH "rock steady" 24 hours a day. I only experience a swing of about .03 or slightly higher through a 24 hour period.

My .02 cents...

Try dosing Vodka or vinegar when your lights come on to keep your ph a bit lower especially if you use kalk.  This might counter the kalk and keep the tank from having such a dramatic swing in ph over a 24 hr period.  Start dosing vodka or vinegar in very small amounts, like a few ml and gradually increase it over a long period of time.  It's a very gradual process and will take time to see results.

I actually think dosing Vodka or any carbon source will drop pH?  It depletes oxygen, which will cause a drop in pH right?

Merk wrote: Try dosing Vodka or vinegar when your lights come on to keep your ph a bit lower especially if you use kalk.  This might counter the kalk and keep the tank from having such a dramatic swing in ph over a 24 hr period.  Start dosing vodka or vinegar in very small amounts, like a few ml and gradually increase it over a long period of time.  It's a very gradual process and will take time to see results.

My 2 cents:
Keeping Alkalinity in range and then moving the water from bottom to top as fast as possible is all it takes.

An RDP Refugium is useful but I've had about 10 tanks since my last RDP Refugium tank in 2007 and they all worked great growing SPS and everything else really well.

My last bit of advice is to stop worrying about pH. I have not checked pH in any of my tanks since sometime before the year 2000. It's just not that big of a deal as long as Alk is in range.

That's great.
I never have liked Oceanic salt. It just never has seemed right to me to have extra Ca in the salt mix when Alk is so terribly important.
Is the Alkalinity staying higher now?

I suspect that the reason pH is higher is because of the teeter-totter effect of Alk and Ca. Pouring more Calcium rich Oceanic salt into the tank with each water change was pushing Alk down and with it went pH.
This is the beneficial buffering effect that comes from anything which helps Alkalinity. As far as salt mixes go, I believe the same improved effect would happen with the tried and true standard Instant Ocean salt mix. This is my opinion, of course.

From what I said above about not worrying about pH, out of curiousity, my followup questions would be:
Have you noticed any difference in the tank?
Are coral happier?
Are fish brighter?

The corals are happier and even my badly sucked in candy cand that it's skeleton is covered with coraline algea.  It's pushing out and trying to be bushy again.  I see the colors on my corals to be a bit brighter and polyps extending more.  As far as fish I haven't noticed a difference other than my chocolate tang having a spine of my urchin in his side this morning (puzzled) - Hopefully he heals ok.

FYI, Not too long ago, IO salt in the 200 gal mix was $45 with free shipping from F&S. It comes as four unmarked bags in a box.
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I picked up four of these boxes for a similar price from The Aquarium not to long ago.