Help with high pH

[FleeVT]

pH is swinging from 8.5 to 8.66 over 24 hours. I am using a pH meter (calibrated 3 weeks ago) and have verified with the Hagen High Range pH test. I have tried to slowly bring it down with Vinegar, Soda Water, and Baking Soda on separate occasions, but it always goes back to the high range after a couple of days. I've read through this article and have to assume it's an aeration problem? I have a 72 bow with a 20 gallon sump/fuge and a Mag 5 return. Skimmer should provide aeration, along with my 2 Koralia 3's. That's a lot of turnover. Is there anything else I should be checking? Thanks for any help.

Rest of parameters are good:
Ca 400
Alk 4.0
SG 1.026

[Carlos]

Do water changes and the pH should come down.

What is your dKH?

Carlos

[FleeVT]

Converting meq/L to dKH yields a dKH of 11.2.

I do 10-15% water changes every week with RODI mixed with Reef Crystals. I'll check the pH of the fresh water to see what that is.