Runaway pH

[baldgeek]

For some reason, over the last couple of weeks, my pH has started going up and up. First, the basics:
210G reef, not drilled w/ 2x1200 GPH overflow boxes
3x250W metal halide, 2x160 Super actinic
45G sump (the sump light has been off at night lately to try to lower the pH)
pH=8.3 at night, pushing 8.5 during the day as measured by ReefKeeper 2 (freshly calibrated) and confirmed with titration tests
"Normal" for the tank would be 7.85 at night to about 8.15 during the day, at least that is what I have always had
Nitrate=~10
Ammonia=0
Nitrite=0
Phosphate=<1
KH=10
Calcium=490*
*I have had the calcium reactor turned out for about 1 week as I thought this might have gotten to the point where it was buffering the water so much the pH would not come down. This has caused a small increase in the pH, but there was just too much calcium in the tank.
There are no others additives/dosing going on aside from top off of RO/DI via an AquaMedic aquadoser in one of the overflows. I tested the pH of the RO/DI with 2 different titration tests and got 7.0 & 7.1, so I don't think there is anything leeching into the water from the container. Of course the water sends the probe into orbit, but the titration tests both agree.
I can lower it by way of seltzer water, but that seems temporary as it always goes back up again.
I did a 50G water change yesterday that seems to have had very little effect. The pH did drop a little, but not more than .01-.02.
I am really at my wit's end with this, so if anyone has any ideas at all, I am open to it. It was a little cooler here last night, so I opened up the house to vent a bit thinking that after 2 months of barely a window being open. That had no effect either.
I would really appreciate any thoughts anyone has on this as I don't know what to do. As an aside, the corals in the tank don't seem particularly bothered at the moment (and this has been going on for about 3 weeks), but I can't help but think I am heading toward a crash.

Thank you,
Mark

[Boomer]

First, you can not measure the pH of RO/DI water with a pH probe, only a kit. Second, the only thing that can drive pH up as high as you are getting is Algae or something in the tank that is dissolving like DIY live rock not properly cured. Third, there is no issue with that pH as long as it doe not go any higher. Lastly check the pH of the tank with a pH test kit and probe at night and during the day. I see maybe a probe that has algae on it giving false high pH readings. As, algae at the probe water interface will pick up CO2 and raise the pH at the algae probe interface.

[baldgeek]

Thanks Boomer. I realized that about the probe and RO/DI when I tested the topoff, so I have been testing it with a kit as well, and it seems to be the same as what the RK2 reads. I will double-check again though as the lights do not come on until later in the day.
I really do not have any sort of algae to speak of and no DIY live rock. It did not occur to me until now that maybe the probe just needs to be cleaned, maybe distilled vinegar?

[Boomer]

At last resort yes vinegar but make sure it is well rinsed off in RO/DI. pH 4 buuffer is a better idea.

[baldgeek]

Ah, I had not thought of a low pH buffer. I should also test the pH of the seltzer. I know it must be pretty darn low, but I am not sure if it is as low as vinegar.

Thank you Boomer.

[Boomer]

You bet :)