Sharp drop in pH in DWC grow - why?

Water at 25 degrees has a ph of 7.0 (neutral) Water at 100 degrees has a ph of 6.1 The temperature.

The plant cannot selectively take up some things and not others... if it fits in through the roots it goes in. So, the plant should not impact pH of substrate based on what it drinks. is the reservoir covered? dark? this will help with stability. Also, good products are well-buffered, meaning they will be more robust at avoiding pH swings. I can leave my rez in 80F for 10+ days, build up a biofilm and it still has a stable pH. this is the difference between a fertilizaer company that hires a chemist and one that does not. I am not saying anything specific about the product you use... but if others have similar issues or not? the writing is on the wall or it is not... causality doesn't give a shit about our preferences or feelings. Depending on nute levels, oxygenation of the liquid can raise pH indirectly (involves interaction with certain molecules in fertilzers), but that has to outmuscle the CO2 that is going into solution just as fast as the O2 from atmosphere. CO2 will reduce pH as it goes into solution. Microbes are the other concern. The question above about being sealed and dark will help greatly here too. do you have a cleaningn regimen? do you use sanitization products, like h2o2, or beneficials to control your reservoir, like great white or whatever it is called? ( btw active microbe in that is much cheaper than what they sell it for. Southern AG has a concentrated product that has same bacteria in it) So, it could come down to cleanliness if no actions are taken in this regard... microbes can cause significant shifts in pH. keep rez dark, keep it cool, and keep it enclosed. you'll have to clean it less often that way and less likely some opportunistic infection gets a foothold. give it a good cleaning and full refill vs topping off unelss you can eliminate some possibilities and see a better path.