Ph drop in water reservoir (drip system)

thanks for the explanation.. Given it's a drip-only system (and the water doesn't come back into the reservoir) it can't really be usage of the plants (unless the water is moved back, which i don't think is the case based on your explanation and the fact that it's in soil). Even then it's more expected to drift upwards instead of down. When (not if) water evaporates in the reservoir it will change the nutrient-to-water ratio, and that can lead to PH drift as well, but yet again, i would think it should be moving up and not down. I would say this is what you mentioned yourself with "usually it goes up". I assume you're not using RO water, are you? If using tap water, then there are also some impurities, that can bind themselves to what's in the ph buffer in the nutrients. Also i take you leave enough time in between the various nutrients and the PH down to let everything stabilize so you're not measuring a "hotspot" (or the opposite)? In the past i had something similar, but that was due to a bad/very inconsistent PH meter, but that's more of a corner-case, so nothing i would usually expect.