Water changes and PH

I just did a water change a couple of days ago. 4 gallons. It seems that when I do a water change, my PH drops like a rock. It was a steady 8.3, now, at night it is dropping to 7.7. During the day it goes up to 8.0. Here is the crazy part. My sps corals seem to like it. I resisted adding any buffer, but I have added a small amount to try and get it up.
Now some important facts. ca=390,KH=13.4,MAG=1500,ALK=4.80,PH=8.0 at test time 8:30PM
My D.I. water is 9.5 PH out of the tap. I use instant ocean salt. My ALK is high from the buffer that I added. I have cut back on the co2 to my reactor, my ALK has always run a little high about 4.0 average. I have been trying to get my CA up. Now I am going to back off and go for a more stable ALK and a CA of 390. Seems to be where the CA wants to stay.
My main question, why the sudden drop in PH with the water change? With a ALK of 4.0 going into the change,D.I. at 9.5,salinity at 1.024,protien shimmer running 24/7(good for o2) What am I missing? My only thought is that the reactor is keeping the ALK too high. I have good water movement at the surface. I don't think that I have excess CO2. I think that I am going to cut off the CO2 for now and see if it will settle back into the norm. Note; the CO2 shuts off with the lights all the time.When things are all OK, the PH is 8.1 at night, 8.3 by midday.
Special note: I just received a shipment of sps frags from pacific east aquaculture(drmaccorals.com) Although the shipping was a little high $66.29 plus $10.00 box fee(weighed over 30lbs.). I received 12 frags that most are at least 2" tall, multi branched,mixed colors and types of acros. Realy steller pieces. I have never received corals packaged as well as these. I would have had to pay $40 each for most of these at my local pet shop. I am quite pleased.

 

First off Sweet SPS deal do you have any pictures yet (up in Canada we don't get as nice SPS as you in the US)
I would agree with your thoughts on the Ca reactor. My first though was perhaps aeration of your water change but only 4G in a 70G tank shouldn't have effected it that much.
Have you thought about dripping Kalk as well as the reactor. This will stabilize your PH and aid in your CA numbers. You could also build a Reactor effluent cup. Just a simple PVC cup that your reactor effluent goes into then fills and flown into the tank. this gives the effluent a little more time to come back up to the correct PH.

Would love to see pics of the Tank it sounds wonderful

Jason

 

I don't have a digital camara to take pics.Sorry. What would be a good calcium addative without any buffers, and it will still show up on my sailfert test kit? I just do not have any room for a Kalk drip set up.Maybe I can get my ca up that way.