I have checked the pH (and the EC) of my nutrient solution dayly for a period of Â± 2 weeks, and have found that the pH and the EC increase. The ph by Â± 0.05 points per day (ie: from 6.5 to 7.0 over 10 days) and the EC by about 0.025 (from 2.000 to 2.250 over the same period)
The higher conductivity is a result of the plants (Swiss chard) using more water than nutrients.
The increase in pH must be because the plants tend to absorb nitrate-ions in preference to Ca, Mg, or K-ions.
This has led me to adjust the pH with 4% nitric acid as a compensation for the nitrates rather than the usual "pH-down" acid, which is either sulfuric acid, or phosporic acid.