Trouble regulating PH

[rwoolley]

I have a 75g reef set up for just over a year now and suddenly my PH is dropping and I'm having trouble bringing it back up. Any thoughts, what should I be checking? Thanks for any feedback. :?

PH 7.68 (electronic monitor, just re-calibrated)
Salinity 36%
Temp 79deg F
Magnesium 1250 mg/L
Nitrate 2.0
Phos 0
Ammo 0
Nitrite 0

 

[cdeakle]

My first though was Magnesium but looks like your ok there.....

What is your alk??

 

[rwoolley]

Alk is high 12-13 dhK. Everything seems fine, but this is pretty low ph isn't it?