Zandor has posted recently that undernuting could cause PH drop too. But he's at around 1500 so that seems unlikely provided everything is calibrated.

I sure couldnt figure out why mine just drops, but it does. I havent used any PH Down since I changed to RO water. With my plain tap water, I had to down it like crazy though.
MisterE Reviewed by MisterE on . Drop in pH..but good growth...???? Hi all. I'm about 3 weeks into vegging and my girls look great. Lush and healthy. I've been experiencing slight drops in pH like from 6.0 to 5.7 or 5.7 to 5.5 during the light cycle. I'm not sure what it means or if it's fine or what. TDS at the last rez change (5 days ago) was about 1700 with all nutes and supplements combined. The plants took very well to it. TDS today is 1370 so the plants are using up the nutes. Usually when plants use nutes the pH tends to increase a bit, not Rating: 5