Algae and other things growing in your
nutrient solution can cause pH to change, but I cant say up or down for sure. However it would need to be a fairly good amount, or small water volume to really notice. The water temp itself shouldn't change pH, but could cause something in the water to grow more rapidly. Though if your using a electric meter to test your pH, water temp will affect its calibration/reading (check your manufactures instructions). The amount of mineral salts in your nutrient solution can change the pH, and if I remember correctly you said your EC/TDS/PPM rises. If so, that may be the problem right there. But because I cant remember, or keep straight the details about everybody elses setups, I need to ask some questions before I have any ideas.
1. Exactly how much does the pH change regularly?
2. What
nutrients are you using?
3. How often do you change your nutrient solution?
4. Do you completely flush the system between nutrient changes?
5. What
growing medium are you using?
6. Are you using a
pH meter, or drops to test your pH? And if your using a
ph meter, are you double checking your readings with pH drops as well?
7. What pH adjusters are you using?
8. Are you adding any additives to your nutrient solution?
9. Are you spraying the foliage with anything that might drip down into the growing medium?
10. What type of water are you using for your nutrient solution.
11. does the pH ever reach a certain point then stop rising on its own anymore?
12. Does your EC/TDS/PPM rise along with pH? And if so do they rise and/or stabilize together?
13. How many gallons of water does your system hold?
14. How much water is your plants drinking daily/weekly?
15. Are you replacing the water the plants drink with straight water, or nutrient solution?
16. Does it look like there's anything growing in your nutrient solution.