Well, been 5 days now on nothing but RO water and the PH is still dropping a few tenths per day. Initially I added about 30ml of H2O2 to the res (10 gallon full capacity), and only PH Up since then. PPM is 30. I've needed to add 5ml of PH Up per day, approximately. Been topping up with plain RO water as needed.

Plants are growing, but I'm still kind of amazed that the only reason I've seen posted for PH to drop is too much nutes and nothing else. I can't even find another post anywhere here with a "ph dropping" issue.

Maybe I should just run half RO and half tap water, to make use of the PH buffers in the tap water? Or maybe 1 gallon out of 10? Any input on that idea appreciated as well.