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.