Reef aquarium ph

[Bfwaits]

I am having some difficulty with the ph in my reef tank. My sump ph is 8.2 to 8.3. But my display ph is 7.4 to 7.5. I don't understand how they can be so different and why I can't get them to equal. Please help with suggestions.
Tank stats
125 gal display
35 gal sump/refugium
Mag 12 return pump
Pc lights on 9 hours a day
Sump light on 15 hours opposite display
Had lights at 12 and 12 with same ph results.
100lbs lr
120lbs ls
Kh 9, ca 350,
Thank you for any help

 

[mr_X]

how about the amount of gas exchange in each tank? water movement and surface agitation effect the ph. i never thought it would effect it like you are explaining though, but maybe so.

 

[ccCapt]

Welcome to AA!
I have to think it's a testing error. If the pump is doing 900 gph (taking into account head loss) your sump is turning over aprox 30 (30g x 30=900g) times per hour. I think it's impossible for the pH to rise from 7.4 to 8.3 in the little amount of time it's actually in the sump. I think your getting a bad pH reading when your testing the tank. If it really was 7.4 I doubt anything would be alive.