Strange behavior and I have no definitive ideas, but I can put forth a wild theory:
If you have poor circulation in your tank, but decent skimming, you could be removing suspended micro algae that produces oxygen in the system. The resultant CO2 buildup from poor circulation/gas exchange would cause your
pH to acidify.
What kind of circulation do you have in either tank besides the
skimmer?
P.S. Like I said, wild theory.
