Your Ca and Alk look good- maybe too good.  Which brings me to this question, 500 Ca is tough to keep without either a reactor, or almost daily supplementation with a liquid CaCl2 supplement and your stating that a water change actually brings it UP!????????

There is no salt premix that has anything even close to enough Ca in it to actually BRING up the Ca concentration in a tank to 500+.  I am going to go out on a limb here and guess... your test is bad, or your doing it wrong...  or there is something else your adding to your water your not telling us :)

Seriously- very very few aquariums run at 500+ calcium that do not have hundreds if not thousands of dollars invested in reactors and controllers-  even running a concentrated lime water dip as ATO would not generally keep your Ca that high.  My advice is to actually go to a store, pay them to test everything, then if its actually what your test is telling you, I would seriously consider letting it DROP by stopping some of the additive you HAVE to be running.  500+ is a TON of calcium for the "average" tank, unless your running a very serious SPS dominated tank you do not need to be anywhere near that high to get great growth.  I have a fairly SPS dominated tank which is producing very nice growth, I never even bother worrying about Ca if its close to 400 and I am fairly certain I have never once seen a Ca reading higher than about 440 and I dose about 7 gallons of nearly saturated lime water as ATO EVERY WEEK by continuous drip.  My tank eats it all, I have no (as in ZERO) precip of anything in the water.  

Just decided I made to many mistakes typing this so quickly at work.  I want to help, not hurt so here is a condensed version.  I did not mean to offend anyone by saying that if your stable at 500+ Ca your simply not using it up... but its just a very very high number to maintain without supplementation which appears to be the case... 

 Sea water by virtue of the carbonate alkalinity =  carbonic acid and bicarbonate and carbonate, buffers much more against RISES in pH than it does against drops. in the pH range we care about, and given your very high Alk and Ca its going to be even harder to push it up, just give it time without adding things and it really should even out.

Also, at high Ca you can enter into a pH/Alk combination that promotes precipitation of Ca Carbonate out of the water.  Complete precipitation of ALL carbonate would only marginally drop your Ca levels.  So- I think 500 is really high and this combined with the Alk pH buffering phenomena could be part of your problem.  If your seeing alk change a lot day to day but your calcium appears "steady" remember that at 500+ Ca you can remove by precipitation ALL of the carbonate and change the Ca level by only little bit, (would have to do the math but it would almost certainly remain WELL above 400)... this would of course almost certainly stress the tank beyond the capacity of any corals.  So, this is why its better to keep things more in the middle of the range, I don't know of many individuals that advocate for over 500 Ca, its just too high and brings with it too many issues. 

My comment about water changes was simply this, if your salt is mixing at above 500 Ca, and your water changes are bringing you over 500Ca, unless your making really really regular water changes of LARGE percentage - its obvious your not using a lot of Ca up.  So, since over 500 is really well outside of a normal level- I would recommend you try to see what can be done to bring things to a more typical level.  pH is mostly going to be influenced by the C02 exchange with air and the concentration of C02 where the aquarium is (house, work whatever)- you don't have much to say about it (I completely agree with Mark do NOT add anything to directly influence pH).  

I guess All I am saying is that things seem just a bit out of balance- try to bring them into a more normal range.  Ca and Alk are linked, and there are MANY posts of very experienced aquarists that regard Alk levels in the range your testing to, which is linked to your Ca level in part, as TOO high.  

Everyone does things differently, I figure the ocean has it pretty much figured out, so for me I try to hit over 400 but dont worry much past that in Ca.  Alk at that level is going to be lower by a LOT than your reading.

I will say this, the only time I have ever killed something by a direct consequence of an action I took, was using Alk supplements and chasing a value I didn't need to chase. Its hard to be sure what causes what, but for the most part I would just stop with supplements if they are being added because of a pH reading.  Then I would wait, see how it goes for a week or two and test WEEKLY.  Go slow, don't make radical changes, think about if you really want to be in the 500 range for Ca given the attendant Alk levels your seeing- most tanks run more like 400-450 with Alk 6-8 or even a lot lower than that.  

I get nervous when I start approaching the edges of "known" safe parameters.  

Yes- Oceanic is REALLY REALLY high in Ca.

 

Just curious but occasionally i add a little baking soda to up the dK reading in my new saltwater when i am mixing it. Is that a bad idea? I don't add much but I like to keep my alk around 9-10.