forgive me if this has been mentioned, but do you have any kind of alkaline buffering in your mix? like potassium silicate? i've seen these kind of wild pH swings (and constant nose diving of pH) when the initial water doesn't have enough buffer. as much as i don't like recommending it because it's a PITA to work with, using something like protekt or southern ag's offering makes a lot of sense when starting with water that has no alkalinity to it.
basic res change out process is:
- fresh h2o
- add 2.5ml - 5ml/gal protekt
- let mix for a solid 15-30 minutes
- pH down to 6.5
- add nutes
- allow to mix for 10ish minutes
- final pH down to target
it makes the process way longer, but having that alkaline buffer (the silica will drive the pH up to 9+) then bringing it back down to 6.5 offers a hell of a lot more stability to how the pH behaves unless your EC is way out of whack or you have a bad bacterial issue due to anaerobic conditions. bacteria will drive the pH down due to make the solution more acidic when it blooms. that's also why i run sterile. trying to cultivate live cultures and keep them healthy is a nightmare without a chiller, and even with a chiller if you end up with pythium or worse, you're never getting it completely out of the system.